With things like this sometimes they do the work without preliminary perf numbers because other browsers have already done it successfully. IIRC, at least one production browser already does off-main-thread parsing.
That's not to say Servo shouldn't try it of course! Part of having a healthy multi-browser ecosystem is each browser trying lots of ideas and coming up with new solutions to problems that were encountered in other implementations.
[1] http://trac.webkit.org/changeset/162260/webkit (the number cited here is a joke :P)
[2] https://groups.google.com/a/chromium.org/forum/#!topic/loadi... (see also the design doc linked from the email)
WebKit/Blink put fewer parts of the HTML parser off the main thread, so negative results should not be taken to apply to more comprehensive approaches.