I have exactly the opposite problem, wasting away my MTT winnings trying to learn the cash game, I rationalise this by remembering how few successful tournament pro's there are compared to the many, many cash game pro's that make a very good living.
I don't think there's any hard and fast rule about not mixing the 2 formats but its worth remembering that just about all the top tourny pro's made their money at the cash tables, so its probably me thats learning the game back to front, whereas you can finance your MTT's by your ring winnings i have to get lucky in an MTT to afford to play ring games ( or deposit again ).
To summarise, you could cut out MTT's and make a fair profit, or take the odd MTT and win a nice wad but if I don't learn the cash table game the
odds are that my
bankroll will never get much bigger or could disappear altogether, hope this helped coz it just confused me more lmao