Six buttons are mainly to cover the fighter genre (SFII, Killer Instinct, etc.), which are mostly, if not all, horizontally oriented. It's not just overkill, it's completely unnecessary as you'll never, ever use them. You really only need three if it's all going to be vertical games.
Granted, you can "play" horizontals on your vertical, but it's going to be compacted and cramped and sub-optimal. If you're looking to play these types of games, consider switching to horizontal display or planning a second cabinet.