Stock is NOT as fast and optimized as pure Vanilla is on the G2. The difference is pretty big in favour of Vanilla. Try playing the Final Fantasy 6 remake on stock G2. Framerate from hell and random reboots all over the place. On Vanilla it runs pretty much perfect on the G2.
However stock has its very strong points featurewise that you have to give up when going Vanilla.
I agree with OP though. Today's new phones give us less and less reasons to flash custom ROM's except perhaps overbloated carrier specific variants. I bought a Sony Z Ultra the other day too and I will keep that pure stock definitely. Sony's UI on the other hand is very optimized and not very far from Vanilla in smoothness so I'm pretty impressed by that. Sony also have a very beatiful UI across system & apps which I unfortunately can't say for the G2. It's a mixed bag of OK design and pure crap ala Touchwiz design. But featurewise it's awesome and has more features than the Z Ultra.