Well what I have is this

Priest:
/stopcasting
/targetparty
/cast Dispel Magic

This cycles nearest to furthest party member. When I see debuffs pop up on my guys I spam this button. When I look at my preist 99% of the time I see nothing to dispel. When I look at my DPS meter at the end of few dungeon runs I Might see the priest has dispelled a few things.

My shamans have

/stopcasting
/assist emrict
/cast Purge

Same thing with them when I see beneficial stuff on mobs I spam this button. Sometimes they go away sometimes they do not.

I guess I was thinking that dispel would remove all the debuffs my guys get. What I learned after doing some reading is that is is really for boss mob fights or PVP.

Same for Purge. Most of the buffs/debuffs tossed around in trash fights cannot be dispelled or purged. However when I see alot of boss mob stuff get cleared off my guys that are usually DOTs of some sort. For example the second fight in ToC the memory summoned gets a renew put on it that heals a decent amount and it can be purged off.

The really dangerous things like posion and disease the shammie totem takes care of so for the most part these are a luxury in PVE and probably a must have in PVP.