It should be obvious, but: Competitive mode is about competition. Incentivizing random players to join competitive matches for credits diminishes the over all quality of the competitive environment. (inb4 hackers did that already.) Rather than playing against players who have made the conscious decision to play competitively against other like-minded players, you are now instead paired up with and against players who have been lured into playing the game mode purely for mission-profit.
Missions should be mode-neutral, and achievable regardless of play-mode, allowing both competitive and non-competitive players alike to complete missions and earn credits.
