You know, I just don't know who to believe these days when it comes to browser benchmarks. All browser vendors pretty much say they lead all browsers in _something_. Honestly, I don't even know what all these benchmarks mean most of the time.
Here's how I think browser benchmarking should be done: https://blog.mozilla.org/nnethercote/2014/06/16/a-browser-be...
I've certainly written JS that builds trees, modifies them, tears them down, and builds new ones, all within the same event loop turn, and so would certainly benefit from incremental GC in the cases when the data it's working over is very large (or it's just a part of an app that's already been burning through a large number of allocations).
[1] https://code.google.com/p/octane-benchmark/source/browse/tru...
It may not benefit the app but it may benefit the user, which is clearly a relevant metric.