We've fielded questions on this a number of times, so I thought I'd write a little about the decision here.
Firstly, we don't have an infinite number of engineering hours. We're not language builders, and we need to focus very clearly on building the datastore, machine infrastructure and management tools that benefits our customers. Secondly, the Mozilla team are going to be building around SpiderMonkey for the forceeable future. Improvements like Tamarin only benefit us (and projects like IronMonkey may allow us to run Ruby, Python & PHP code in a hardened environment too). While we'd love to be able to contribute back to Mozilla eventually, right now we can't afford to.
Announcements like today's about TraceMonkey validate this decision further. We'll be able to benefit from this change very quickly -- we've already got a version built in development, and as soon as it hits a stable point, we'll be able to roll the change up very quickly.