Medium term, two r2 summons will get you a ton of mileage in boss rushes.
Due to the way Ether works (it refills either one of your ability slots at random - 1 charge if the ability has 1-5 max charges, 2 if it has 6-11 max charges, and 3 if it has 12 max charges) Summons will never ever get the benefit of recharging two shots per ether. However, chucking two r2 summons with Summoner RM2 means you're guaranteed 3 summons per battle in every stage, as Ether will always recharge one or the other.
Going beyond r2 is a risky proposition considering the ever higher hone costs, but I think once you have a few of each elemental weakness done to r2, there'd be nothing much else to use the Orbs on anyway. Like anything that comes in limited quantities, though, I'd only craft/hone when it's required.
My personal target medium term is an r2 of one 3* and one 4* of each elemental weakness, e.g. Ifrit + Phoenix, Ramuh + Ixion, etc, since that'd cater to most bosses and spread out the summon orb cost more evenly between 3* and 4* Summon Orbs. Valefor r2 makes a nice stopgap for a lack of a particular 4* element though, since it's got a reasonably high multiplier.
p.s. it takes a single Major orb to take Phoenix or Valefor to r2 compared to 6 Major Fire each for Flare, Flare Strike, etc. It's a drop in the bucket, really.
