The momentum of minecart's is capped and they can no longer store momentum.
Sure this led to some op-ness but I, personally, classed it as a feature (as did many others) of minecarts...
I think if anything the feature should not be removed but rather nerved. So there is a cap for stored momentum rather than having no stored momentum...
Demonstation:
http://www.youtube.com/watch?v=bE7b67MmWmM
Sorry, this is the intended behavior now 🙂