I think alot of it depends on how well the series it's competing in is known. I don't think Chad DiMarco and the Legacy Turbo helped Subaru enhance their "performance" image a whole hell of alot in the early 90's, yet he kicked some ass in it. Why, next to no one ever heard of Pro Rally( at least those who were potential Subaru buyers).
With the growth of import performance cars in the last five years, and the growth of Pro Rally(which is directly related to the growth of import enthusiasts, IMHO), and the internet, it's alot easier to reach out to individual markets, thus, SOA felt it would be worth it to run a Pro Rally Team because many of the people buying the WRX are VERY aware of the car's presence in the rally world.
Now, hopefully road racing imports will continue to grow, and continue to get coverage from Speedvision(or whoever) as well as the magazines and whatnot. Then I can see it helping the brand image.
Many guys want a WRX because they see the WRC Impreza going absolutely crazy in the forests and mountains around the world with fire shooting out the exhaust and cherry red brake rotors and hear that wicked Subaru exhaust note screaming like a mad banshee. No, it's not the same car, but it's "image" is the same which is what is so appealing to a prospective Subaru buyer.
It will be a long time(if ever) until a road racing WRX ever has that impact on someone. Sure, the road racing *helps* a bit, but if we are talking in terms of what Subaru thinks is of the utmost importance, I think rallying was, is, and always will be the sport to "enhance" it's brand image.