Preview for waveform changes in Multitrack

Yes it has, but not for Multitrack view. No it isn't turned on by default because most of the time you don't need it. It's very easy to turn on though and yes, it will do what you want:

Nobody's ever asked for it in Multitrack view - it would clutter the screen up too much. But if you double-click on any file in your session, it will open in Waveform view and you will then be able to see what an effect your effect is having (but you have to apply it in Waveform to do this, obviously).

Of course the other reason that you don't have this in Multitrack view is that until mixdown, all effects are playback only, and don't affect the waveform, as such, until they are finally applied as a part of the mixdown process.