Lecan is on the money on this one. My wife (WHM 75) will sometimes sub SMN depending on what she is doing. Having SMN subbed gives hers that added bonus of Auto Refresh and additional MP. Wouldnt be a bad piece to macro in when summoning.
I don't know if this particular piece can or cannot be equipped by all jobs, but if it can, it isn't as crazy as you people seem to think. SMN is able to be subbed, like any other job. This adds a bit of usefulness to Carbuncle, either subbed or main. It also gives a benefit to the caster for having Shining Ruby up, which can help while soloing.
Does anyone know if these gloves affect all summoning cast time or just Carbuncle? I would imagine all.