Yeah, no. They chose intensive process, many of them are ports of code from other languages and some the tests are using software made by Mozilla such as pdf.jz and zlib (Ecmascripten) and Microsoft (Typescript compiler)
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.