The UMP would be more practical, but I think the tommy would be a great firearm for our troops nowadays. Fired one a while back....yeah, heavy, but it was plenty accurate, recoil was nonexistant (muzzle rise excepted), and all that fun jazz. Granted, drums still have the same reloading problems, so it'd probably stick with sticks, but think about it---if we were to go for the WWII version instead of the intricately complicated closed-bolt-firing weapon as it was designed, but with use of modern metals, machinings, and plastics for grips (maybe use the navy-issue folding stock?), and you've got the makings of a good weapon. Better SMG purpose than an FA M4--becuase why? Because it is.
I've seen pics of our troops raiding houses with antiquated soviet PPSh SMGs, because they didn't think that the M4 was adequate for the job. That seems to me to say that "WE NEED AN SMG NOWADAYS".....and if we were to apply modern methods into a tested & honed design, whose to say it wouldn't be a good firearm, either for noncom personnel or, say, close-quarters combat, etc? Because no matter how you look at the issue, the M4 using FA is NOT the thing that fills the hole SMGs left in our military.