Well, to make Str + Wis effective, you need to keep up two scores. Easy to do at 1st level, very hard the higher you get. However, if one score gets added twice, it's much easier to get a lot of bang for your buck.
Also, we really aren't talking about 2*Wis vs. Str+Wis. With Marked Scourge it will be Str+2*Wis most of the time in paragon and epic.
If it didn't stack with Pit Fighter, then whatever. It's really, really good for a feat, but it's not mind-blowing. For people who would probably take the feat, you're looking at at least +2 damage on every attack against a target you've attacked in the last round, which I guess you could say equates to around +1 to +1.5 damage per attack. Stacked with Pit Fighter, though, I'd venture that you get a little too much return for your investment, when compared with other feats.