Well, a Lamborghini isn't very practical either (ever try to carry a sheet of plywood in one?) BUT THEY'RE A HECK OF A LOT OF FUN! Same with full auto guns. If you have the resources, by all means, buy one and shoot it.
Really, after buying a semi and the ammo you'll shoot over, say, 5 years, compare that to the price of a full auto, and you may be surprised to see that a full auto isn't totally unreasonable after all.
I have a number of semi's, and a number of full autos, and I can unquestionably say the full autos are more fun to shoot. But the semis have their place too.