The question is not whether guns have a place in D&D, but what place they have. They normally don't feature in traditional fantasy (the D&D-inspired version of it, at least) and I wouldn't argue to have them on the Players Handbook equipment list alongside the crossbows. But traditional fantasy has always been a starting point for D&D, not the alpha and omega. Guns are entirely fitting for any game being run in a post-Renaissance/industrial setting, and there are various ways to work them into a traditional game if desired. I've heard that the DMG might feature 'medieval guns' (to work alongside the standard equipment list), and 'post-medieval guns' for games where guns are the primary ranged weapon.
They are, of course, entirely optional, but the DM running a D&D game with guns in it is still playing D&D, not Pathfinder, WoW, Castle Falkenstein or some degenerate nitrate-laden D&D simulacrum.