The summoner's summon monster ability doesn't work while he also has his eidolon summoned (which should be almost all the time), so you wouldn't lose anything really important and instead you would gain a very powerful healing ability. That would be even more overpowered imho.
If you want a "melee buddy" to throw at enemies and be able to heal the party, you could instead play an alchemist: the mutagen lets you fight pretty well, although not as well as a synthesist summoner, and you get healing spells.
A summoner with channel positive energy would be better than the alchemist in all possible ways, hence the "overpowered" mark.
One way to get cure spells is to be a Samsaran and take those spells with Samsaran Magic from the bard's spell list. Channel would still be limited to paladins, clerics and oracles of life though.
Or just play a cleric... channel positive energy takes care of your healing needs, and you can still summon monsters.
Or a druid, lesser healing potential but you get a pet, almost as powerful as the eidolon and you can also summon more pets...
Or a cleric with the animal domain, he gets both the pet (although less powerful) and channel positive energy.
