There's no doubt that the pros can still beat anyone else with any racquet, but that doesn't mean they're not obsessive about their racquets.
Pete Sampras was, he hired Nate Ferguson to customize and string his racquets exclusively.
From an interview with Nate:
"The company I was with in Connecticut was almost strictly custom-building racquets, so we had a great list of clients, including Lendl and Navratilova, and along comes this 19-year-old kid named Pete Sampras," Ferguson says. "He called and had a lot of questions about racquets and I explained to him what the process was of duplicating a favorite racquet. He had a real genuine interest in racquets and how they’re customized. I could tell the handle, head and feel of the racquets were very important to him right off the bat."
As rafazx10 said, they care about it when they're at the beginning of their careers and still trying to find the perfect setup for their game. Once they've done that, there's no pint in changing it anymore, they just keep it and use different PJ's.
Probably that's one reason why they are so reluctant to talk about their racquets in interviews, since what they're really playing with is totally different from what the sponsor claims it is - sometimes even a different brand.
Wilson BLX ProStaff 90: 368g, 7pHL, SW=352. (Wilson NXT 49lb / Lux AluPower 45lb).