GMMichael
Guide of Modos
D&D does seem to allow an alternative way to fight if you use the non-combat rules, but it's pretty hard to see given how the combat rules are woven throughout everything else.I guess this is not what you're looking for, but for me D&D itself offers at least two combat systems: TotM and the full default combat system . . .
I know many DMs who use TotM for short, unimportant battles and the full system for featured encounters. Personally I even occasionally skipped the combat rules completely and reduce trivial encounters to a couple of rolls, making something up in terms of random possible costs for the PCs. Even if you just decide not to use HP for smaller monsters and let them die at the first hit, in a sense you're using an alternative combat system.
D&D might be a good example though. It seems you're referring to the DM(s) making the decisions on which system to use. How do the PCs feel about the choice of system? Or more specifically - how do the PCs feel, given the type of characters that they are playing? Are the non-warriors glad to see a TotM battle? Do the warrior-types feel that they're missing out if not all combat rules are used?
Would you let the PCs choose the system at the start of combat?