Titan heart is like a 1min "burst", you have time to get your dots up multiple times so it's really not a burst mechanic. Garuda adds are a lot closer to a burst requirement that makes summoners meh, but even then it's still 20+ seconds. Not saying summoners don't need some buffs, Bane aoe should be way way bigger for example and that'd fix a lot of issues with the class, but overall from what I've seen summoners do really good sustained damage especially on movement heavy fights(they're not bard but then again who is) and they have quite a bunch of utility, most important of which is an instant incombat rez, since they'll have the mana to spare unlike healers on tough fights(unless again, bards).