Hi,
as far as I remember we kept MPC backwards compatible since it was a reasonable effort and it enabled users on Helios to consume new features in MPC. I don't know to what extend users or integrators actually install newer MPC releases on older Eclipse versions though. It wasn't a requirement that was requested by the community but something that we wanted to provide to target a larger audience.
Backwards compatibility is always a trade off between consuming newer platform features, effort required and target audience. If there is a significant benefit in moving to Kepler and high effort to continue maintaining backwards compatibility I would drop support for releases before Kepler. As Martin pointed out integrators can always consume older MPC releases or backport the current release to their targeted Eclipse stream (and contribute that back).
Reducing the number of Eclipse targets also reduces testing overhead and if we have a very good reason (which sounds like it) it seems fine to do this late in the cycle.
Steffen