In Red and Blue, a rare glitch can occur, where he will use a Super Potion on Pokémon he just sent out despite them being at full health.
Generation 1 was a screwy, buggy mess. His bad AI likely isn't on purpose, but instead a fault in the games coding.
If you want an in-game explanation to why he uses Super Potions on healthy Pokemon, Blaine has always seemed a bit... eccentric to me. He could be mad, and healing his Pokemon when they don't need it. That is of course complete speculation, however.
Hope I helped!