I co-lead my guild with my wife (the official leader by rank) and you will always lose good people in both raiding and casual guilds. The reasons span the entire range of possibilities. As stated its part of the job. It always sucks to lose good people but you make the best of it and move on.
As far as healers are concerned its just one of those things. Healers are always in need. I frequently get whispers asking to guest run in raids due to the shortage of healers. My guess on this is that with the coming expansion many healers are taking time to level up alts (i know i am) since the demands on your time can be high guild wise. I know my healing has been in need for more than one alt dungeon run. I dont say this with any sort of malice as i LOVE playing my priest. I wouldnt have made it if it werent for the guilds need in the first place.
Just make the best of the situation. If it means calling in a few friends to fill the gaps or posting open slots for a raid in LFG channel then so be it. Enjoy the game for what it is and have a good time.
