Oh? How so? Talking too much? Mjoll does that. I'll be finding out info about a new mission and she'll be blathering on about her mother in the background.
I dunno about Serethil, but I can't stand followers because:
1. They like to step on traps.
2. They like to run directly between me and enemies, and then yell at ME for the fireball/arrow/battleaxe to the face they get as a result.
3. They like to block narrow doorways and trap you in corners.
4. Some of them even like to shove you, when it's convenient.
5. They like to run off and fight anything that comes in range, even if it doesn't aggro.
That's just some of the reasons, TBH...