In principle, he's not wrong. Once you have a 'full' set of songs then inevitably any incoming song will result in an outgoing song. That said, if you need three sets of material then the best place to be is to have FIVE sets of material fully-rehearsed and gig-ready, so that you can shuffle the deck and keep things interesting.
On the 'playing all the parts' thing, it's easy (as a keyboard player) to think that you need to supply the horns AND the strings AND the piano AND the Hammond, and no, you can't do it all, all at the same time. But you don't need to.
The audience will be hearing the song in their head anyway. Just so long as you don't leave out any vital hook, lick or riff to snap them back into the real world, you'll be fine. I have plenty of songs where the intro is on organ, the verse is on piano, the bridge is on strings, but all the audience remember later is the horn stabs in the chorus. 🙄