But do you think Live's built in instruments warrant such complexity, latency and instability?
If you have to buy the other sequencer then you can just run its own internal sounds and use vst plugs from the score enabled sequencer.
I think cubase has score writing capabilities and it has similar built-ins to Live.
I wouldn't do it myself as I own Logic and don't care that much for Live's built in stuff, but it is an option if you really want to use Live (which is what I understood from the OP's message). I have routed between DAW's internally plenty of times before and it is quite stable once audio is turned off on the application that is solely transmitting MIDI. I was really just putting it out there as a viable option! Saving the score as a MIDI file is another good alternative, though if you load into Live and then want to edit, it could become tedious.