It's everyone's preference--If you and your friends are alright with that, then go for it, but not everyone is and that personal preference doesn't make them a tool. I know many people who are fine with it and it works out for them, but I also know many instances where it blew up into a giant debacle.

IF it would negatively affect your relationships with the people you care about, are you still willing to pursue it without a second thought? Some people, maybe that girl included, wouldn't.

Sometimes it affects relationships because it creates animosity between the couple, especially if the breakup was a nasty one (i.e. I don't want you hanging out with him/her because I can't stand him/her) and the mentality behind that would be "why is my boyfriend/girlfriend spending time with someone I hate?" And that would in turn create anger among friends "you're going out with him/her now so you don't want to hang out with me." Some people can't let go of the past, so they don't want to hang out with their friends at the same time as their ex because it can be awkward. Other times it's the worry that the ex will go back to the friend because they once had something together.

Lots of messes--that list can go on and on. But it's their choice, and neither is right or wrong for it.