I am a slytherin academic supervising a couple of PhD students now and I endorse the above message. If you finish your PhD, collaboration with others is key to future career development. If you don't finish, though, it's another story --- so make sure you get through. You can always forget to tell your supervisor about some of your collaborations.

If the "mercenary" papers are unrelated to your field, it would also be advantageous to start thinking of ways to portray them as related - this will be helpful in presenting your research as a coherent whole, rather than giving the impression you are just helping on others papers for the sake of your CV. Or, as identifying a specific area of expertise that has cross-disciplinary applications.

Finally if you are not able to portray your mercenary papers as related.... maybe your supervisor has a point and you should work on developing collaborations that offer more than just a one-off publication. I have a couple of "mercenary" publications from my PhD period (no further collaborations beyond one paper) and they stood out like sore thumbs in my CV when I applied for postdocs.