I wouldn't call Zelda a RPG, it's an Action Adventure IMO.

But Kingdom Hearts is for sure. The battles between Zelda and KH are way different.

Well it always depends what you want to achieve.

Action based RPGs like Kingdom Hearts are good to let a mass of enemies spawn and you have great, fast battles, jumping around, slashing here and there, throwing in some magic, etc. Lots of things happens on the screen.
The contra part on this is that you barely can do anything tactic related as you don't have a lot of time to think about your next step. But the fights are way cooler.

On the other side, round based RPG's like Final Fantasy give you way more chances to fight in a tactical way. The game Lost Odyssey for example, IMO the better version of "Final Fantasy", has great round based battles. You don't wait for the ATB to fill up, you just choose your next attacks/magic and a list is generated who will be first/next (kinda like in FFX).
The battles really great too as the bosses are hard and it takes a lot of tactical planning in order to defeat them.
With a combination of great effects, music, SFX and the option to make your attacks stronger everytime you attack (you have to press a button in the right moment, kinda like the gunblade from FFVIII), the battles are great fun.

So i wouldn't say round based RPG's are "out of date". If you don't have hard loading times like in old FF games, and you combine something so the player has more interaction in a fight, it is a great fun too.

The most annoying thing on Round Based fights for me were always that you wont see enemies on the map, but just randomly enter a fight. Games like Eternal Sonata made it different and it's way better like this (also a great round based RPG COMBINE with action elements).