most current pcs already are faster than the current console generation so it's no big deal if javascript consumes the difference.

but i also see the advantage in smaller games. it's extremely cool that it's not necessary to download or install anything. just visit a webpage. for example i very rarely checked out games by people on this forum because i just found the downloading, unzipping or worse, installing and cleaning up afterwards way too annoying to be worth it. (and by the way every game you downloaded potentially was a much bigger security risk than webgl is. it just could have wiped my harddisk.)