Dear Odileph, rainbow decks are usually OP already, they don't need another buff. Furthermore, don't forget that in its upped form Battery ++ costs zero quanta, which means it fits into any element. Also, every element has creatures to benefit from the buff and CC are common to every kind of element.
Consider the following deck:
Mark of Fire (or even Air if you wish)
6x Wind Tower
5x Damesfly
5x Thunderstorm
5x Unstable Gas
5x Elite Firefly
4x Battery ++
If you have, let's say, 2 Damesflies, 2 Elite Fireflies and 1 Unstable gas on field, 1 Thunderbolt and 1 Battery ++ on hand and your opponent has at least a single targetable creature. Play your Battery ++, then your Thunderstorm, dealing 1 spell damage against the enemy creature, buffing your 2 Damesflies and 2 Elite Fireflies by +1|+1 each. Right after that, trigger your Unstable Gas, dealing 1 spell damage to every creature on the field and 20 spell damage to your opponent, buffing your creatures with +6|+5 (6 atk because 5 creatures and 1 player were taken spell damage, 5 hp because your creatures took 1 damage from UG). Right now, in just a single turn, your Damesflies have risen from 2|1 to 9|7 creatures and Elite Fireflies from 4|2 to 11|8, OMG!! Now, if your opponent had 2 creatures on field or 3, then just with a Thunderstorm your creatures would have risen from 2|1 to 4|3 or 5|4 and from 4|2 to 6|4 or 7|5, I won't even bother in calculating UG's buff in that case... (and all this in just a mono-Air deck with a fire mark)