The small guns don't do enough damage to do much to a Dropship, the Blaster couldn't reach, and I think the missile would travel way to slow to hit anything. I think Tanks are better off driving to get the Dropship in the primary cannon's firing angle and taking it out like that. Still probably need a Railgun to do it effectively as the Blaster gives them time to move and, again, missiles are slow.

With a bit more elevation a small railgun would be really useful at scaring off dropships when coupled with the rolling forwards / backwards tactic, the main issue is that with the new fitting cost of small turrets, no one fits them because it gimps your fit, tanks and dropships alike. The fitting reduction from skills just doesn't cut it, even when maxed, so there's not much incentive for it.

Also, I believe tanks are still bugged in that your turret is forced onto the front, and you have to fit two in order to get the gun on the top.