Biggest con:

GFX aside, most game mechanics are relatively simple implementations from a coding standpoint.
If this happens, once the community gets well acquainted with the code it would take less than one year to code in M2:TW-like features and in a couple year it would reach a good enough gameplay that it would represent a serious menace to commercial sequels.

Two examples: Descent: Freespace 2 and Jagged Alliance 2.
Try the open source versions of the games and tell me square that the level of refinement reached in the gameplay can be matched by a company on a budget