I agree that it would be nice to have a third wing who is a better defender than Mason.
Now, the third wing also need to be a very good 3 point shooter. Take aside Bonner, who I hope won't play a lot of minutes next year, Spurs' PG and PF/C aren't good 3 point shooters. So if you 3rd wing isn't a good 3 point shooter, Spurs will play heavy minutes with a lineup with only one good 3 point shooter and it won't work.
There are quite few players who are both very good shooters and defenders. I don't know if Spurs can realistically trade for one. If they can't, I think the best is to keep Mason as 3rd wing and to have a defensive specialist (Bowen or maybe Hairston) as 4th wing. The problem is that Finley is too good and too expensive to be a 5th wing, so Spurs should really try to move him somewhere.