Maybe it's just a switcheroo; we get Cecil for free but have to pay for Tidus, while it's the other way around for the JP servers. Honestly though, I would rather get Tidus for free if that was the case. Cecil is kinda "meh". Way too many better light tanks out there.
Which means to say he isn't that useful in PvP. He's only useful in PvE. He'd just take up an extra slot in PvP and waste opportunities to deal damage.
Actually, Cecil is what we OG gamers refer to as a "spiker." His ability board is full of Crit Chance +%, Luck +, and even his TMR is packed with attack + and Crit chance +. Pair him w/ the right summon / VC combo and he'd be a wrecking ball in GB and AB with the right support. If you're going to use him, play into his strengths. Pad his atk power, crit damage, light atk & slash atk.