It's not about healers throwing out DPS when they can and making a mess of things, it's about having the common sense to discern the appropriate time to do (or not do) so. This is the difference between an average healer and a good healer.
First phase of the final boss in Antitower? Definitely DPS.
On a low level dungeon where the tank can go several minutes without needing a Cure? DPS.
An extreme fight where the boss is charging up/ adds/ a phase there will be no incoming damage for the next however few minutes? DPS.
Everyone's about to take a heavy party-wide hit? Don't DPS.
Tankbuster coming up? Don't DPS. And so on.
With more ambiguous scenarios - i.e. there may be time to throw in some DPS but you're uncertain about the strategy or what sort of damage to expect, then that's your call. You can try it if you feel confident enough, but there's also nothing wrong with choosing to be careful and leaving it until you're more comfortable with the fight. Most people here complain about healers who don't DPS at all, not those who are simply cautious.