FOST is necessary to have the best FO team possible, but also needs a TON of gear. His taunt gives buffs to the team (heals them under zKRU lead) and gives TM to the team, and with his zeta he calls a lot of assists (which also gives advantage to the attacker, and we all know FO likes advantage) and can do so when he counter attacks. He's also tankier than KRU in terms of raw protection pool.
That being said, if you don't want to use all that gear on FOST, the special forces tie pilot is better. He has an AOE that applies a buff to your team (also heals you under zKRU). Under zKRU that AOE can crit 5x giving him 100% TM allowing him to go again and he also a targeted assist that will inflict another stun for the team if the targeted ally was FO (it should be in a FO team). He also hits harder than the standard tie pilot, but I believe is also a slightly harder gear than the original.