I hate being locked to single inheritance. Like, okay, we got the properties of foo; so that means we now absolutely cannot be a bar. It bugs me even if there is no plan to do that in the current program. By inheriting from foo, I'm enforcing the decision that this cannot be a bar, which could cause a problem to a future maintainer (maybe myself in six months).

Java has single inheritance, which sucks; but it has multiple inheritance of interfaces at least, and good luck doing any serious work in Java without that being involved. (That is also designed that way in order to pander to implementation ease and certain run-time efficiencies; but has been turned into fallacious arguments like "multiple inheritance is bad ... unless it's interface inheritance").

Oh, and one problem with this prototype business is that in fact it is single inheritance (in the typical incarnations, at least; am I invoking a strawman)? It seems inescapable that supporting multiple inheritance under the prototype paradigm requires that we make a single new object out of two or more existing objects. But if objects are effectively just bags of properties (a.k.a. hash tables) it does seem easily doable. Make new empty object, stuff with properties of all argument objects, season to taste, return.