Scizor - Worth Powering Up??

It is up to you. If you want a specialist against exeggutor, go for it.
You will not really need it.

Yes, i would vote yes. You never know if these are set THE set raid leaders(might be a rotation.) and also possibly a Mewtwo counter?

Best psychic counter in general, not just Eggy. That is my opinion although people will argue Tyranitar and such, which data supports.

Plus Scizor is just awesome.

I put my scizor in gyms because he looks cool but that's not really a reason to power him up.

I would absolutley power him up:
He is great in the upcoming psych-meta, he has very good defences, he absolutly ddestroys the upcoming legendary Celebi (double effective) and exeggutor. The final reason: there will be never a better bug attacker with good resistances at the same time and psychic legendaries will also appear in the future generations.

Don't waste your precious dust on Scizor! Dust is even harder to come by. I would only dust legendary counters like Golem and Tyranitars. On raids you don't need supreme counters unless you're trying to solo a level 3 raid boss.