Is this Jolteon now worth a power up?

No. Jolteons are really only good on attack. Their stamina is simply too low to ever be even mediocre defenders. Keep it for attacking Gyarados (or even other water types) until you evolve one with either of the other two charge moves.

I guess Jolteon is useless for defense, since he hits 0.8x Rock/Grass/Dragon and are weak to Ground, so Dragonite, Exeggutor, Rhydon and Golem can kill him easy (most common attackers). I would power up one for offense (Thunder Shock/Thurdebolt or Thunder Shock/Thunder) who can punish Gyarados and Vaporeon with dodge. Thunder Shock/Thurdebolt is A for both attack/defense (still weak for defense), i maxed out mine 95.6% today.

I was using a lv30 Jolteon (and Electabuzz) as attackers prior to the changes and they made good attackers. Jolteon is now excellent against a range of Pokes providing you dodge. I used Thunder for the Jolteon and the two bar Thunderbolt with the Electabuzz.

With the buff the gap between Jolteon and Electabuzz has grown to the point I'll be powering up a 2 bar Jolteon.

I'd be tempted to try a Thunderbolt Jolteon as a defender in a strong gym (Lv8 +) as he would force attackers to be cautious in their selection due to the huge damage he can cause water types even if he doesn't last the full battle, but his uses as a defender is pretty niche - he's basically a fast powerful attacker. Discharge is just too weak to offset his low life expectancy.