The "but why" for a .45acp isn't really any different than for pistol cal carbines generally. Without belaboring the usual points against them the pluses would include :
Rifle easier to shoot more accuratly/ farther than a pistol of same cal , especially for less experienced shooters.
Generally low recoil.
Shoot them at indoor ranges that are otherwise handgun only.
Generally less muzzle blast than real rifles for those who do shoot outdoors in areas of marginal noise tolerence.
Ease of supressing.
And the .45 fans mantra of "because it make a bigger hole" is just as true from a carbine.
All that said , I can make a better case for leveractions in .38/.357 , .44mag , and .45Colt , but that's why we have choices.
And for the $ the HiPoint is a good option once you get over the ugliness. But you can't see much of it while you are shooting it.
Another option, although it doesn't take Glock mags is the Beretta CX4 Storm. An aftermarket supplier, Sierra Papa, sells a mag well adapter to allow the use of PX4 .45 magazines which have two advantages over the stock 8045 mags: 1) easier to find, and 2) ten shots rather than eight. Sierra Papa also sells magazine extenders which allow 15 or 16 shots, although I'm not sure if/how they fit into the 2013 FSA law.
Better pointing, faster first hits, fast enough follow up shots, substantially more power, more range.
Even though the above comment was more in the spirit of adressing the comment that .45acp carbines had substantially similar power to a .45acp pistol. The magnum lever gun ( or Ruger level .45Colt loads) is substantially more "rifle like", with option of loading down to more "pistol like" levels with .38+P , .44spl, std or cowboy .45LC.
Hi Points take their own proprietary magazines. There are aftermarket, higher capacity mags but they are complete junk. So you are limited to 10 rounds in 9mm using the factory mags. I don't know about how many rounds of .45 they hold.
The good news is that Hi Point sells their mags direct from the factory and they are not a rip-off. I bought 4 or so extra mags for probably what one mag would cost from certain manufacturers. They are a bargain just like the gun. Because they are 10 rounders you can still buy them on-line and have them shipped to you.
There is a thing you can get kind of like a shotgun saddle that fits in the stock and holds two extra mags right on the gun. Again, adding that is not big bucks.
You could totally trick the thing out with a light, decent, cheap red dot, bunch of extra mags, saddle, and even a bunch of ammo for well under $500.
Personally, I like mine just the way it came - I run it bone stock. I wouldn't take on the Russian army with it, but in any HD situation I think it would be formidable, even with only 10 rounds on tap. Small, light, easy to handle, super-reliable, excellent iron peep sights (they look weird but they work great). You are going to put the rounds on target - it is really easy to shoot accurately out to at least 50 yards or so. Way easier to shoot accurately than any pistol.
Is it beautifully honed and of the highest quality imaginable? No. But it is built ruggedly enough to get the job done. I kind of think of it as the Ford Crown Victoria of the gun world.
I went back and tracked down the ATF ruling that allows RoadDawg's carbine set-up above. It turns out the key is that the stock assembly has a legal 16" barrel length, and when a collapsible stock is added it reaches the legal Overall Length in MD. The stock assembly I saw in Florida was indeed an SBR when assembled.
The ATF link provided by the manufacturer FAQ in RoadDawg's post is a dead end. I went to the ATF webpage and found the ruling and it is below:
The one thing this set-up has going for it is that you can pair your Glock 21 with a KRISS 29-round mag and not worry about reliability/feeding issues. But I repeat, the only 29 round mags that feed reliably are the KRISS mags that use the Glock 21 factory mag as its foundation. All those other Korean gun show 29-round mags are useless.