This ally is so good that I would argue he's completely and utterly broken. Allow me to explain.
First, let's assume for the moment that you always trigger his ability since, as others have pointed out, there's little reason not to. Then, to simplify analysis, we can regard him as a 2/3/1/2 ally. Normally, allies in this game are balanced around having stats:cost ratio somewhere in the range of 2:1 - 2.5:1. Allies who have less than this usually have a beneficial effect to compensate. Similarly, allies whose ratio is above this range typically have some sort of drawback. For example, Gandalf has a very high ratio of 3.2:1, but you only get him for one round. Treebeard has a whopping 3.5:1 ratio, but is unusable on the turn you played him (still OP, but that's a different story). So, how does Quickbeam measure up then? His ratio is 4:1, with no real downside other than being unique and not allowing restricted attachments. That's insane! We're talking about a 2-cost ally whose stats total matches or exceeds 29% of the heroes in the game!
Now let's look at the stat distribution, which is arguably more important that total stats. Does that tell a different story? Nope, still broken! 2 for 2 resources is as good as the best 2-cost allies in , which is the sphere that supposedly specializes in . And 3 for 2 resource is strictly better than all other 2-cost allies in the game, including all allies! We have both of these facts being simultaneously true of a single ally, and he has decent survivability with 1 and 2 .
All of the above assumed him having one damage. If you manage to heal him (easy to do in sphere, e.g. via Wellinghall Preserver), or for some reason you don't trigger his ability, then things become even more broken. Now you have a 4.5:1 ally with even more survivability whose stats total matches or exceeds 60% of heroes in the game, all for a measly 2 resources.
This analysis also only considers him in a vacuum -- his ability and Ent trait have powerful synergy with other cards like Booming Ent, Boomed and Trumpeted, Leaflock, and Ent Draught, making him even stronger in decks with those cards.
I get that Ents are cool and FFG wanted to make them feel powerful, but I feel like they just threw game balance completely out the window with this one. Needless to say, you need a pretty good reason not to include at least 1 of these in your deck if you have access to the sphere. I usually only exclude him when Ents don't make sense thematically or if I just want more of a challenge.