RCP support would be great, but you really have to define what that support is. MyEclipse is an add-on to Eclipse, not a replacement. Eclipse already has the PDE and plenty of stuff regarding RCP. Afterall, even Eclipse itself is really just an RCP app.
Frankly I don’t think there’s much that MyEclipse could add. PDE and RCP problems are, IMHO, most about distribution and branding.