From my experience, when using a mixer, just as with coinjoining, you want from the observers to know you're mixing. Swapping BTC for XMR and XMR for BTC obviously provides better levels of privacy if used properly, but the outputs do not look as if they're private (from a blockchain perspective).

The next level of a fungible currency is for the coins to look the same, not only from the users to treat it likewise. For example, in whirlpool you cannot distinguish which coins comes from which user. Or, you can simply swap BTC for XMR and just stay there, which is completely fungible for obvious reasons.