First of all, Restaurant Week tends to not be much less expensive than regular dining - and much harder to obtain reservations. Certainly some participating restaurants like the Earle or Pacific Rim do present more affordable options, but others - Cottage Inn? Jazzy Veggie? Sava's? - are pretty inexpensive to begin with.
Second, Restaurant Week can rely upon discounted food prices and high volume preparations to make Restaurant Week economically feasible. The most upscale places don't want to lose too much money during Restaurant Week, so they can cut corners in other ways. You may not be getting the quality of food or service that you'd have otherwise.
Even at the higher end restaurants, careful planning and menu choices (plus some saving up, of course) can make Restaurant Week unimportant in the scheme of things. But if you do decide to go, remember to tip beyond what the bill might say. Just because you're eating cheaply (or cheaper) doesn't mean the waitstaff has to lose the money, right?