I don't really see the big deal with an eidolon having an AoMF with this property. It doesn't heal the caster. Isn't there already evolutions that add +d6s of the various energy types? In Pathfinder its even less of an issue, because the Eidolon is getting hit by the AoE priest heals anyways.
The mob tactics should still be the same....Kill the squishy mage and the eidolon goes 'poof'. In our Pathfinder game monsters usually take a swing or 2 at the big scary eidolon, then they just turn on the rogue or the paladin with the 2-hander....they are much easier to hit. The one time our group faced a summoner with his own big scary eidolon, it took us about 2 rounds to realize attacking the eidolon was dumb...and the summoner died one round later when everyone rushed him instead.
Dumb question though....how do you buy a +0 magic item? Don't all magic items have to be at least +1 before other enhancements can be put on them? ie. a +0 flaming sword makes no sense at all. So that means even at +2 you are looking at shelling out for a +3 AoMF.
You can't buy a +0 magic item nor can you make one. All magic items have to be at least +1 and anything with a property is going to be considered a +2 in the least.