I've never run into an opponent on la corr who built in the center. In empty corners, yes. Never the center...
In any case, the use of a "significant ammount" of spiders is mainly to delay attacks. Your opponent sees the spiders, and decideds to build up first, giving you time to build up as well (possibly surpassing them in the process). If they see it as a delay attempt, they will rush in and loose units.
All the talk of the MD's has me wondering if we fixed the exploit yet...