In play, the trap works well stretched across entryways or narrow spaces. And of course, in the process of trapping the adventurer, the trap is also likely to inform its creator (the aforementioned Giant Spider) that dinner has arrived.
Update: The Monster Manual 3 describes a terrain called web sheets (MM3 p.183) which is very similar to my Giant Spiderweb Trap design. It makes an attack versus reflex (which the Giant Spiderweb Trap should probably do to be consistent with trap design) but only immobilizes victims (can't move). I think restrained (can't move, -2 to attacks, grants combat advantage) is more consistent with how a giant web would work (and be more useful to the spider spinning it) but to each their own.