This thing about tracks, wheels, and engineering makes people forget about capitalism economics. Subaru, like everyone else, goes for the money, and the hardcore fans are a desperate bunch.
They knew people would buy the WRX no matter what, and yes, they need to set the STI appart of the other models using performance and looks. Looks are very important, it's what sets a Porsche appart from a Ferrari.
The WRX is probably the one with the most sales, so think about a small change like the wheels. Say the 17 inch is $100 less than the 18 inch, that's $400 per car for, say, 30,000 cars worldwide...probably more. So, that's $12 million less on cars that will sell anyway. Now add track, suspension and body. Oh...they took out the multi-CD player, was that a bad system? No, they had to save somewhere, and there went the wheels too.
What they probably know is that fans will go for spacers, wider wheels/tires, or whatever. And we know Subaru does very tough cars. I say the body and track change was to come anyway, they just got some money upfront.
I'm surprised the space was enough for 25mm, good for the trial and hit.