To me it's simple.
So, first of all, when I tank I use my mitigation skills to keep myself alive and make the healer's job easier, I don't do it so the healer can DPS.
However, if the healer can DPS then go for it, as long as their fulfilling their primary role of healing me and the party it's not a problem. In this case, if the DPS roles decided they didn't like this, I'd side with the healer.
On the flip side, if a healer doesn't feel comfortable DPSing for whatever reason, then as long they're reasonably busy with healing I'm okay with this too. If very little healing is needed they should do DPS.
To sum up, DPS'ing shouldn't be at the expense of healing but healing only shouldn't come at the expense of doing DPS where possible.