Are you implying Presto is multi-threaded?
No (though it will in fact do layout in parallel with JS execution by time-slicing its JS, last I checked, unlike other UAs).
My point is that it's bad if all browsers switch to WebKit and web sites absolutely require WebKit to work correctly, which is what the parent of my comment was suggesting should happen. This point is somewhat independent of what happens with Presto per se.
People have tried to parallelize WebKit and failed: too entangled, too much code making too many assumptions... Parallelism is one of those things that's just incredibly hard to bolt on after the fact.
So if you want browsers to never really scale well to large number of cores, by all means try to get them to all switch to WebKit.