Zodiac, there is a big difference between finishing a game (killing the final boss) and completing a game, especially if you aim for 100% on everything.

I don't cheat on games, but I do abuse glitches in the programming, such as levelling up Marcus to raise Eiko's stats (FFIX) and using the W-Item duplication (FFVII).

Regarding your last point I prefer FFX, but FFX-2 isn't without merit.