First, why all of a sudden is my website "that website"... like as in The Website That Shall Not Be NamedTM or some such? I'm not the friggin' Voldemort of Michigan Highways for crap's sake...
Because it’s a website that was referenced?
Well, you had put it in boldface and referenced it as "that website" for some reason. It was like we couldn't say the name of it, but needed to highlight the fact that we were calling it "that website." Maybe I misinterpreted why you were stating it like that. It was just weird.
Is it as safe as it could be? Definitely not. Would it be worthwhile spending the extra dollars to have made it a truly freeway-to-freeway interchange, at least for the US-31 through movements? Well ask anyone injured or killed there by people driving across the painted white lines or being confused by where to go, especially when it's snow-covered and/or dimly-lit (or just being an idiot behind the wheel). You might have your answer to that.
Place a barrier over the double solid white line.
Which, as has been stated, creates other dangers. Then we're to the point of spending money to swap out Danger No.1 for Danger No.2, which may not be a wise use of taxpayer dollars.
It's... sufficient. I guess. 
I agree, it’s a sufficient interchange that operates adequately for the traffic served.
But, again, sufficient and safe aren't the same. The former set-up with traffic using Napier Ave. to shuttle between I-94 at Exit 31 and US-31 at Exit 24 was "sufficient" to create continuity in the route of US-31, but was it safe? Was it the appropriate configuration, especially long-term, for the traffic served? No. The old two-lane US-31/33 route the entire St. Joseph Valley Parkway replaced was "sufficient" to get traffic between South Bend and Benton Harbor, but it was also a death trap, hence the freeway we now have. Sufficient isn't what we should settle for if there's a better solution is what many here are saying. Why build a vastly superior freeway to replace the old death trap highway only to end it with an interchange that's marginally "safer" than its predecessor?
Second, as Terry mentioned, I'm the undisputed expert here. Period. Just take a look at the banner on most pages of my website. It says it's been online for a quarter century now (which is a lot more impressive sounding than just "25 years"). A percentage of my site visitors weren't even born yet when my website first went online! And that doesn't make me feel old, no matter how much my daughter keeps telling me I am. Anyway, with MDOT sending people to my website and County Road Commissions constantly linking to it and even the Michigan Attorney General using it in legal opinions, that pretty much says I'm a friggin' expert. And the video I took I did at my own expense... meaning it kept me from enjoying a Lou Malnati's deep dish for an extra 15 minutes that day. So that should tell you something. (And anyone that can't enjoy the humor from this preceding paragraph needs to lighten up.) (But I'm still the expert. Terry said so...)
Nice?
Indeed. Anyone who has met me will verify that I am definitely nice. Just look at all the Michigan highway-related stuff I put on the Interwebs for free! And my maps are pretty nice, too, ya gotta admit. So, there's really no need for the question mark. Maybe an exclamation point would be more appropriate for the situation...

(e.g. lighten up!)
A "barrier" here would not allow for sufficient shoulder or escape room, making it dangerous in a different way. That's assuming a "permanent" barrier, like a jersey barrier.
There’s a full right paved shoulder, and a barrier installed would allow a 2-3 ft left shoulder. How is this any different than say, an overpass bridge, with a 8-10 foot right shoulder and a bridge wall?
First, modern standards (from the past several decades) mandate shoulders on
both sides of a lane of travel, not having traffic up against a jersey barrier,
especially on the outside edge of a loop ramp just a couple feet from said jersey barrier. Throw a little ice or snow into the mix (we get a sh*t-ton of that here in the Lake Effect Snow Belt where this interchange is located) and there would be absolutely no room for error with zero left-hand shoulder. A vehicle bouncing off that jersey barrier coming to rest in the lane of travel at the top of a blind loop ramp in snowy conditions or the dark would really cause problems without a full left shoulder that would/should otherwise be present here.
A barrier address all of the alleged safety problems without any issues.
Actually, it would not. In addition to the issue of the zero left-hand shoulder on the
outside edge of a loop ramp as noted above, a permanent barrier would leave the other lanes with zero shoulders, period. You'd have: ONCOMING LANE || LEFT-TURN LANE | THRU-LANE |B| LOOP-RAMP LANE (where |B| = jersey barrier), so you can see, there would be no out for any of the eastbound-to-southbound traffic at all. It's either swerve into the oncoming traffic and hit them head-on or swerve into the jersey barrier (with zero shoulder, remember) and come to rest in the travel lane. So, you've now replaced a serious safety issue with an even more egregious one. Please don't forget the sheer amount of snow we get around here. We have to drive it in daily for several months, plus it also has to get plowed somewhere, too, so that's a major factor.
However, the current configuration is less safe than a truly physically separated movement or than the originally-proposed full-cloverleaf design for this interchange. Like I said in my other post, it's "sufficient" but "sufficient" and "safe" aren't really the same thing.
A physical separated barrier can easily be installed, with adequate shoulder remaining. A cloverleaf interchange would introduce weaving movements for southbound US-31 traffic. Having a channelized lane, and if you installed a barrier, would actually be safer for southbound traffic.
I have no idea where all this "shoulder" is coming from. We've already noted that the painted separation lines are only a couple feet wide. How wide is a jersey barrier installation? A couple feet? Where would you get two full shoulders out of any of that in addition to the jersey barrier?
As for the weave-merge issue, my contacts at MDOT and FHWA have told me the much larger loops in the SW and SE quadrants were designed to leave room for weave-merge LANES to limit the amount of possible conflicting movements:
http://www.michiganhighways.org/maps/US-31Freeway_BerrienAlts.pdf. Thus, you would have two full shoulders on the through (ebd BL I-94 to sbd US-31) movement and two full shoulders on the weave-merge section adjacent to it as well. That's a lot more safety than you have in the current configuration! Just sayin'...