Cecil's LB at 6* does baseline +50% to atk/def/spr for 3 turns. If you maxed it out to rank 25, it would increase to +70%. Leveling a LB from 1-25 on a 6* unit would take an eternity of grinding, and is generally not realistic. The catch is, you can partially level it while he's lower level, which takes a lot less time per rank (they carry over when promoted).
So really, the choice is do you keep Cecil at low level for 1-3 months while you work on his LB, all so you can get an extra ~10% stats for a few turns, or do you level him now and get immediate use from a strong unit. Also keep in mind there's much better sources of that buff down the road, and they don't stack (for example Ramza's +100% all stats song post-enhancement).
Warrior of Light is excellent after his 6* upgrade, and can replace Cecil, but they both have perks in different scenarios, and you want both leveled to use as needed.