A close cousin to the raid leaders who get thte twitchy strategy changing finger every attempt, the raid leader who can't just make up his mind as to whom to bring to the raid each night poses similar challenges to the morale of the raid team.
Put yourself in this position:
You're in the middle of doing a 5-man, or your dailies, or whatever it was that you had planned to do with your WoW time tonight. Then out of the blue, your raid leader asks you to please come and help on Boss X. You agree, and are summoned in so fast you don't even have time to read a refresher strat. Which is fine since after one go at it, you've got a new role to play (i.e. you are now the OT or a healer -- whatever your OS is.) An attempt later, you hear over Vent "Sorry Bud, we are going to bring in X instead" and find yourself removed from the raid.
A. Log out of the game in full nerd rage.
B. Sit in Org for an hour pondering what you did wrong
C. Stick pins in your raid leader voodoo doll
I've seen all of the above (well except for the voodoo doll -- people tend to keep that quiet) happen in this totally avoidable situation. As a raid leader, you need to be well versed in the strategy you are using for a boss, and know, in advance, whom you need to make that happen. If you think you are going to want someone to come in with an offspec to facilitate that, you need to tell them in advance, give them a couple of minutes to prepare by watching a video or reading a strat from that perspective, and give them a couple of attempts to learn that new role.
It's incredibly insensitive to ask someone to drop what they are doing to come help you then not give them the respect of the ability to give it a good shot by not allowing them time to prep. Chances are your weary raid team could use a bio or make-an-adult-beverage break anyway.
When raid leaders make a habit of this behavior, they may find those last-minute calls to help out will go unheeded. Which makes sense. No one wants to help out someone who treats them as a disposable cog in the raid machine, versus as a person.
It's a fine line to tread between being responsive to changing raid dynamics and asking folks to fill a variety of roles and being a jerk. How has your guild managed to achieve that balance?