They do a lot more harm than good to the terrain- and if we're talking near spawn, whoo boy, you can expect to accidentally blow up at least one person's building through trial and error, and that's no good- and can only aim at a predefined spot. It's like being able to attack a creeper with a bow and arrow, but you have to hold perfectly still. You can't adjust your aim either by looking elsewhere, or by moving your body. The creeper has to walk into your cross-hairs. Also, between the time you click to fire your shots and the time those shots are actually fired, there's a seven second waiting period. So, you have to wait for the creeper to walk into your cross-hairs then politely ask it to hold still while you kill it and hope it complies.
But wait, there's more! Making anywhere from one to several mob grinders for gunpowder gathering isn't much of a problem, but here's yet another: someone has to constantly be willing to just sit there and man the cannons all night. If they're not using a mod like Zombe's sun control mod, I hope they've got not just the patience to sit still for hours on a video game they could otherwise be having more fun on, but also sharp enough eyes to spot the enemies out in the darkness.
TL;DR For all of the reasons listed above and more, TNT cannons are a highly impractical solution to the hostile-enemies-at-spawn problem. Personally, I've taken on well over one hundred creepers with nothing more than a sword and come out victorious, sometimes even fighting multiple enemies simultaneously. After you make a set of armor, as long as it has at least 30% durability left, the world's yours.
