is it worth upgrading from 14 AWG to 12 AWG
In my opinion, no.
Additional Capacity
If you need the additional capacity on a single circuit, you'd be better to simply replace that single circuit, or add an additional circuit.
Additional Outlets
If you're adding additional outlets (I'm talking outlets (lighting, receptacle, etc.), not just receptacle outlets), consider installing a new circuit rather than expanding an existing one.
Technology Upgrade
If you're replacing the wiring anyway (removing old knob &amp; tube, etc.), you'll want to install 20 ampere circuits with 12 AWG conductors where appropriate. Though it still may not make sense to simply install all #12 wire.
Free Wire
If you have a ton of 12 AWG wire laying around that you got for free, and you just feel like spending hours pulling cable. Go ahead.