One key difference here is that this keeps Servo open to continuing to be a research vehicle for new technologies without the exposure of compatibility for millions of users. If we ship Servo in a product that does change the equation a bit.
I think part of what pcwalton and metajack are getting at is that Servo isn't "just a research vehicle" but rather intends to keep seeking adoption. I don't believe adoption and research are nemeses -- in fact, I'd say adoption is a core part of Mozilla Research's MO. I talked about this a little bit in a talk I gave this summer:
