This discussion does come up regularly, and I personally have changed my opinion over time, from reading some of the responses.
My initial objections were the cost, but the fact of the matter is, that most governments providing BDM sources, are charging you for them, and at a higher rate than if you signed up to Ancestry (which I haven't). From there, it feels wrong to go to Ancestry for sources, only to "steal" them and use them on competeing sites. But fact is, most of the sources they have been given, have been given to them to freely distribute on behalf of the places who actually hold the sources. What you are paying for, is probably covering costs and providing a small profit. Once again, cheaper than what the people holding the sources, charge for them. So long as you only copy sources off Ancestry, which aren't copyright, which most of them aren't, it shouldn't be a problem. As will be mentioned though, the only real problem after that, is people listing in sources on Wikitree, that Ancestry is the source, and not listing the actual details of the source, including where it came from, i.e. a local archive somewhere.
As for FS, my only personal objection to it, is the beliefs of the Mormon church, which I have a theological problem with, and therefore I don't want to give my personal details to them. Otherwise, it sounds like a fantastic place to have on your source goto list, because it is free. But, once again, if you list them in a profile, make sure you are actually sharing the details that they provided, rather than just saying that that is where you found the details.