Before the 2.5 series, the Grapple Hook had no "ammo" limitation and was "offhand", meaning it was not a device you had to switch to in order to use, but could be fired separately at anytime, regardless of what weapon you were currently wielding. This all made the grapple hook an interesting and dynamic gameplay feature, but it also created such severe imbalances by making players so mobile, that it was only available to be turned on as a Mutator (it was off by default).
Now in the 2.5 series, the Grapple Hook is becoming an integrated part of the game, thanks to the added limitations of having become a pickup weapon that consumes energy cell ammo. This is certainly an improvement, as the GH is an interesting aspect of the game and deserves to be integrated in a balanced way.
But the exact implementation of this feature could be better. There isn't any reason why energy cell ammo should be the limitation for a mobility feature, there's nothing about the crylink, electro, nex and HLAC that makes them more dangerous when combined with GH mobility that should make them more effective than the ballistic or explosive type weapons. And as such, maps with energy pickups balanced for energy weapon usage have the unintentional side effect of fueling GH mobility.
I believe the most logical and simply and fun solution is to make the Mutator Grapple Hook consume Jetpack fuel. This is the most logical because both are about mobility, not firepower, and both add a similar kind of mobility to a similar extent. This is the most simply solution, because it doesn't complicate map balancing by mixing up ammo limitations with the already diverse energy weapons. And it is the most fun, because Jetpack fuel regenerates on its own, so that it is an integrated part of your native mobility options, while not being so unlimited and powerful that you have to use it all the time to be competitive (assuming the Mutator is active for that server).
Then, in the future, when and if the Jetpack becomes integrated into the non-Mutated game with its own separate fuel pickups, the Grappling Hook can use these as well, instead of the energy ammo.