I love missions where you're just expected to defend a spot for a certain amount of time. I actually just played a mission in Starcraft with that underlying theme and loved it.
Are you talking about that early Terran in the not-BW campaign? That was brilliant. It's the only defence mission in any game ever in which you can survive for over half the time without doing anything. Really. I started it up, then sat down and read a book, and managed to survive for 16/30 minutes. But other than that one, I also love defence. Unlike most RTS campaign missions, defence missions force you to act, rather than just massing units in your base and then attack moving. Ironically, the Dawn of War 2 campaign managed to get around that by removing unit building, but also made the defence missions a pain in the ass by sending massive numbers of ungodly deathmachines at you (venom cannon Warriors and Seer Councils, I'm looking at you). I must admit, it did make those occaisonal Primarch defence missions where everything goes right and you somehow manage to win pretty spectacular, but if you make a single tiny mistake the alien hordes overwhelm and annihilate your pathetic defenders.
EDIT: Oh, yeah, the topic! Escourt missions can go die in a hole.
No-one likes them, so why do they keep in putting them in? Particularly annoying for me was the Land Raider escourt mission in Dawn of War: Winter Assault. Basically, you had to bring a super-tough transport with vital personnel through a gate that was on the other side of the map. It had a pre-planned course that sent it straight through every single enemy and obstacle on the way, which, fortunately, could be avoided because your commander could call it to his position. Unfortunately, if you didn't click the call button every 10 seconds or so it would decide to go back to the pre-planned path. Great fun when it takes something like twenty minutes to finish the mission, most of which is clicking that damn button so it doesn't bugger off on its own. I haven't played many other escourt missions, but damn that one was annoying.