Caffeine's problem and virtue as a common stimulant is that bodies adapt to it readily and become dependent. Blood pressure doesn't spike when you're used to it, sleep is less effected, etc.

Caffeine's problem as self medication for ADHD is that as a result it's more of an addiction than a self-medication. The short and long term benefits are minimal and it's more of a comforting habit than treatment. Maybe ADHD peeps are chasing that initial benefit, or maybe we just use it because it's legal and we're tired from staying up late.