If you are unsure if you want to spend the time getting your squad promoted and yourself promoted to 1st Lieutenant, here are some of the bonuses you can receive upon promotion.

Squadron members receive chemistry which gives them a chance to bring back materia (DoL and DoH materia. Haven't seen combat materia yet. Do not know which grades).


Access to Priority Missions - Can do one per week.
Priority mission rewards include (in sets of 5 each):
Priority aetheryte tickets. 40% fee reduction for 120 minutes
Gear maintenance manual. 30% gear wear reduction for 120 minutes.
Rationing Manual. 15 minute increase on food effects for 120 minutes
Spiritbonding manual. +3 spirit bond for 120 minutes
Engineering manual. 20% EXP from crafting for 120 minutes
Survival Manual. (same as above for gathering)
Battle Manual. +15% EXP from battle for 120 minutes
Gold Saucer VIP Card. +15% MGP for 120 minutes
Priority Seal Allowance. +15% increase in Grand Company Seals from Grand Company duties.

Each Priority mission requires 540 or 560 points in at least one of the categories and 2,000 seals to deploy the squad.