Well, for one, ... when I am paid in bitcoin, it is my employer who is paying the cost of acquiring (or exchanging to) bitcoin.

For another reason, ... because they can choke our access to the exchanges, but they'll have a lot harder time limiting our access to bitcoin when they need to force employers to no longer pay us in bitcoin.

And one big reason I personally want my (currently) fiat employer to pay me in bitcoin is that then makes the employer more interested in accepting bitcoin for payment (or otherwise earning in bitcoin themselves).