Three potential probs with a triple:

1. cosmetic. A crank can be a beautiful piece of metal. Unnecessary bumps and such would bother some people.

2. chainline. the two outer ring positions are likely farther outboard on a triple, and getting the chainline as in-board as possible is the name of the game in a conversion.

3. Rub on chainstay. If you bring the crank in by using a shorter spindle, you may find that the attachment points for the granny gear will hit the chainstay. Of course they can be ground off to avoid this issue.

jim