I cannot stand them. They were Resident Evil 4's worst feature, they are not fun in any RTS yet they continue to show up, and nine times out of ten they feel shoveled in and useless.
Actually, it's not just any defense missions. It's defense missions like in RE4 where she cannot help herself at all. You basically have to guide an unarmed idiot around who continuously walks into enemies. Escort missions are the worst, especially when you're punished for a teammate dying on you. That's what I loved about Bad Company. Your teammates are relatively smart, heavily armed, and invincible without seeming overpowered. If the people you had to escort were more like this, I probably wouldn't have a problem with them. Also tower/base defense missions in RTS games like StarCraft and Company of Heroes aren't very fun. You have to stay zeroed in on one place for 15 minutes while a barrage of bad guys waltzes down the road and you have to blow them up. To me it's not fun at all.