Optional party members. Remember how in FF7 you didn't have to get Vincent? He was sort of a secret. An awesome one, at that. I want more stuff like that in JRPGs. Though, to be fair, I don't play that many JRPGs so that might be common, just that I'm inadvertently playing all of the RPGs that don't have that.

you talk like you haven't played a JRPG since then, there are tons of JRPGs that have this, and some have even more optional characters than others.

I've played plenty, but like I said in my post I haven't really encountered one where this is true.

you should check out Star Ocean till the end of time.

... That's... Not what I mean at all. I mean members that are secret, and don't have to join your party unless you go out of your way to find out how to add them to it. Like Vincent and Yuffie in FF7 (Though if I recall correctly, Yuffie was actually sort of hard not to get.)

Final Fantasy's always had optional summons. That's kinda the same thing, only cooler. Optional party members are probably uncommon because it would be difficult to work them into the story.

That's true. I loved the optional summons in FFX. And yeah, it would be hard, but I figured since they managed to do it fine in FF7 they should atleast give it a shot sometime.

You should give Shining Force 2 a go then, It has tons of optional characters, and when I say tons, I really mean it, over 10 of them xD.

Because, I don't know, this might just be a haphazard guess, secrets are fun and add replay value?

Of course it should. A secret quest that gives you no incentive to complete it usually isn't all that interesting. A new weapon isn't all that interesting. A new party member is. I refer once more to FF7 where both Yuffie and Vincent were cool additions but completely optional.