Paris Roubaix is most certainly a "b-bike" day for most of us. We would never spend our hard earned cash building our dream machines only to crack our frames and taco our wheels on such a brutal route. The pros feel the same way, however their reasons are not financial, it's more about survival.
You won't see any carbon rims with low spoke counts, or super light frames. What you will see are standard 32 or 36 hole rims, double butted spokes and 23-25mm tires. Last year George Hincapie rode a specially designed Madone frame with a rear elastomer suspension which allowed about 12mm of travel to absorb the cobbles.
Stefan Wessemann's T-Mobile Giant was equipped with Cantilever brakes to prevent mud caking, and Erik Zabel chose an aluminum frame last year over his normal TCR carbon model. Zabel also opted for a shorter top tube to give him a more upright position. Phonak scrapped their carbon frames for aluminum as well. Credit Agricole chose the Look 481 frame and beefier HSC4 fork over their normal Look 585 with all carbon (including dropouts) super light HSC5 fork. Thor's frame had Le Viking emblem painted on (too cool not to mention) and the team chose 25mm tubulars.
Paris-Roubaix is the kind of race where any mechanical or even a flat at the wrong time can ruin your day. If I were a pro mechanic (if I won the Lottery this would be my vocation), I would be like a kid at Christmas on the morning of P-R.