My feeling is that it's difficult to compare team and individual sports, which is really the nut of the best-of-five vs. best-of-three issue. It seems to me that women's soccer is a highly competitive team sport, filled with full-length matches that more or less go down to the wire. And it was wise of the women's soccer establishment to embrace the same structure and rules as the men's version, and nothing we've seen or experienced throws that decision into doubt.
There are two big, interrelated reasons for why tennis is different: tradition and, for lack of a better term, realism. The tradition in tennis has been for women to play best-of-three (for better or worse), and I'm not sure we've seen anything that suggests that this is somehow a curb on the women's game, with roots in outdated views of women as the "the weaker sex." While we've seen many women's matches that could have been tantalizing five-set propositions (the Justine Henin vs. Venus Williams US Open semi comes to mind), we've seen far, far more that suggest that playing best-of-five would just prolong the agony (the Justine Henin vs. Svetlana Kuznetsova US Open final comes to mind).
Tennis is a sport dominated by a handful of individuals, on both the men's and women's side. However, because there fewer upsets in women's tennis, and women, unlike the men, don't play with enough basic power to level the playing field between good and great players (that is, a mid-level women's player can blow a top player off the the court the way her male counterpart can), the additional set(s) would be a critical factor in far fewer matches.
The best-of-five in not a playing-field leveler, which, if anything, is what the women need. It is a way to protect favorites and superior players, on the theory that the longer a match goes (or the more matches a pair of opponents play), the more chances the superior player has to win. Best-of-five is an upset killer, not an upset maker. The WTA did experiment with best-of-five (in the Viriginia Slims Championships final), and while it was an interesting effort, it did not leave anyone feeling that they ought to do it more frequently. And don't forget that television executives, the drivers in today's environment, would be loathe to broadcast two-plus hours of Serena Williams waxing someone, 2-3-1.