Ignoring many of the problems associated with supporting multiple js engines in webkit, there is a lot of work to support any js engine.
Most DOM bindings are automatically generated, but the scripts that do this (and do it right) are large, complex, and completely different for each purpose (there are dom bindings in webkit for AO, V8, Objective-C, and gtk). Moreover, the existence of> 2 sets of bindings should be a sign that supporting an additional set of bindings will not be too complicated (mainly time, not mind-f * cking).
JS , - ( script ..). , , , , . , V8 webkit , , -, , spidermonkey, .