I haven't gotten my healing job yet, but have been in several parties where the healer was DPS-ing and the party wiped because of it. As a BLM, I've had to self-heal on a number of occasions, usually due to the healer not healing at all.

Oh and let's not mention that time the healer decided to pull agro and tank an entire mob in Haukke and as a result kept yelling about her low MP while the rest of us kept trying to get her to heal. Tank died, other dps died, and I ended up taking out the boss by myself because the healer refused to Rez the others because and I quote "The boss only has 10% health, I'll take it." Needless to say she died too!

But then on the flip side, I've had parties where the healer has been absolutely amazing and nobody ever dropped below 90% so it's a bit hit and miss!