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.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.