SP Goku BLU is a free Fighter that anyone can obtain in the shop with Rare Medals. Despite being free, this Fighter is actually pretty good--a must-have for new Players and a good Synergizer for the Saiyan PVP Bench.
He can play an important role in Saiyan and Blue teams. However, his stats aren't that great. The biggest reason to use him in the core line-up of a Team is his Main Ability or as a replacement for one of the tougher Fighters.
Damage Buffing Abilities
Goku's Main Ability increases all Blast Damage the Enemy receives by 40% for 25 timer counts and only takes 15 counts to charge. This means he fits perfectly in any Team that is centered around Blast Damage.
On top of this, many of his Damage-centered Abilities are Enemy Debuffs, meaning they benefit the whole Team.
Powerful Spirit Bomb
The Spirit Bomb is an excellent Special Move that has a 30% chance to inflict Faint while also increasing Blast Damage the Enemy receives by by 15%. Landing one of these bombs will create a considerable advantage against the Enemy
While his Health and Defense stats are decent, everything else is low and overall uncompetitive--likely because he's so easy to get.
Abilities Don't Buff Strengths
While SP Goku BLU works pretty good in Blast Damage teams, he's terrible in Melee Teams. None of his Buffs can be applied to Melee Fighters, making him dead-weight in this sort of Build.
This wouldn't be so bad if his Base Strike Attack stat weren't considerably higher than his Base Blast Attack.