I've had 21 Visas and MCs open at one time, none with annual fees. Much older and wiser now, I only have two or three and most have fees, but all have affinity programs. Who has time for all the junk mail associated with multiple cards? Unless you are planning to max out charges and advances and then leave the country, stick to a few cards that you can regularly use to maximize your miles or points.
A point of interest from someone who worked a couple years in the charge card industry...the credit and charge card issuers figured out in the late 1980's that college grads would stick with cards they had prior to graduation, so they started marketing to college students aggressively. In the 1990's they also went after high schoolers. I'm not surprised at Randy's results for the general population, but I am surprised that FT'rs, who in general try to manipulate the system to maximize their mileage and points would not apply this to their credit cards. Maybe most that stay with the same card are also those that don't pay it off entirely each month and are therefore in a continuous debt cycle, forcing them to keep the card.