/castsequence dps,moredps,whatever
/cast purge/dispel/spellsteal

If you spam that you'll (attempt to) purge after every non-instant spell and it won't interrupt your castsequence, although if your sequence involves dots it will delay their refreshing when you get a purge/dispel/spellsteal off.

Also, are you sure you're dispelling the right target? With the exception of mass dispel you need to target whoever has the buff/debuff you want to get rid of. For example, assuming you're always targetting/assisting to an enemy, the above macro won't ever attempt to dispel a dot on your team.