From 35 to 43 I was pretty much banging the heal key as soon as I could between every other spell. There was no battle that didn't require multiple heals, sometimes just barely allowing my pet to live through the fight. However, I won 99% of my battles. It required a good bit of wandering around and finding what were the weaker dark blues of course, just because it was dark blue was by no means an indication that my pet could tank it even with chain healing.
Quite frankly, even with the 44 heal I've had to spam it during battles. Only since I've got my 49 pet have I been able to heal only once or twice during the battles, it's always been the case before that during every fight I was casting a heal the second I was capable of doing so (in between nukes and rains).
Adding a heal at 34 would make leveling 34-43 a little less stressful, maybe. It might up the variety of dark blue mobs that a mage could effectively solo in that range too. These would both be good things. If it never went in though, I think mages would survive.