Thermal Expansion is getting loads of new content. It would be really, really dumb to take it out.
Also, UE in Tekkit is not going to happen. That's what Voltz is for. (CanVox's own words- Personally, not a fan of UE so I support this decision.)
These are our options as I see them:
Keep Buildcraft and TE. We have been promised a converter built in, and really. Think about it. Is there any reason whatsoever to take it out for 1.7 because of 1.7?
Same as ^ but add IC2. New content is fun.
Tekkit isn't specifically meant to have 1 energy network, it's meant to have a slimmer set of Mods that won't take ages to update. *cough* redpower *cough* The long wait for TE3 is a bump in the road but isn't a disaster.