I personally like the idea, but have a few major questions:
What'll happen with -6 shots?
Will bots still die if they have 0 of all three body methods?
What values would an average bot we see today have? (That is, what level of muscle would an average bot start with, and what amount of cellulose would a plant have? 1000?)
How useful will muscle be? What about cellulose?
Also...
Zinc: Basically to regulate high levels of energy. 32000 energy isn't a wild dream for all bots. Not to mention -1 shots won't steal fat, so it provides some additional defense against -1 shots.
Eric: Unless I'm mistaken, I think muscle efficiency would mean that it would merely increase the output of how much energy is inputted. For example, I'd imagine a bot with 20 muscle would have to output a lot energy to be a devestating combatant, while a bot with 32000 muscle would have to spend much less. And your example does indeed have flaws; spending energy into muscle is far more specialization then just having energy, and would be an additional upkeep for the whole multibot. This could allow for some unusual multibots, as well... imagine a musclebot lugging around a fatbot and using it to just store energy! And take shots from behind that it'd otherwise suffer.
Additionally, if we go with muscles, may I add a suggestion into the mix?
Anywhere from three to five sysvars. Maybe even one. Perhaps we could limit this as a simulation option, because having just one of these would allow for more specialization and thus increase the usefulness of being part of a multibot, while five would definately lean torwards singlebots.
Basically, it'd direct the focus of your muscle torwards one action, basically giving that action slightly more benefit from the muscle, but everything else a bit less. Of course I haven't quite worked it out yet, so it might not be all that feasible, but I still like the idea.