Yes, armor is worth it.
I usually have two stilletos on my interception/tac mission bases, and two sarracen on production bases, all equiped with ecm, targeting comp and armor (no alien).
Armor will make your ships slower, that's true but it's really not a big deal to loose a single fighter just because it got too far away, normally you get a chance to intercept even with the differences in speed, and in the later stages of the game when you have almost the full earth covered in bases speed for interception is completely useless, I can track any ufo over all land masses and intercept wherever I want, whenever I want.
Dropships are to drop on tac ops, nothing more, you can even use them as decoy to tease some fighter ufo and lure it into a trap (sometimes the game tries to ambush your dropship, poor bastards then try to engage the droppy and I just blast it to hell using escorting ships... they will never return fire, they just go for the first ship they target...move that ship around and that's it).