[Jeff Lee]

The cost issue of going larger (50-55mm) is somewhat secondary to what the racer may be really after, and that is having a billet cam core. The reason racers go for a billet cam core is it is may be more compatible with the lifters he is running (Schubeck, tool steel or other non-OEM but allowable materials), and is less prone to flex or twist. This transition of course is a result of the extended RPM ranges you see in Stock.
So the thought is, if your going to run exotic lifters, might as well have a cam core that is more appropriate to the lifters. And if you run a larger core as Dwight Southerland stated, you receive the benefit of a better ramp. And if your going to do all of this, might as well straighten out the cam tunnel (which often frees up HP anyways), and then roller bearings as then you have less concern about wiping out babbitt bearings because you have a ton of spring pressure.
Is this for your average racer? No. But no doubt it's more common than you may think, especially in some of the faster and highly competitive ranks. Nobody said Stock was supposed to be a budget class. And if you are on a tight budget, there are still ways to be competitive and still enjoy racing.