It's a bit naive to talk about refusing to volunteer for extra duties. From my experience nobody truly volunteers, as in going to management and saying 'Please Boss, the MATS pt 2 needs rewriting, I'd love to do it'.
What happens is that management asks you to rewrite the MATS Pt 2, usually with a sweetener like 'You're the only one I can rely on', or 'It will give you a broader insight into ATC management'. Refusing such a request is not simply refusing to volunteer - you are effectively saying 'Nob off', and you will probably find that EG's and days off suddenly disappear.
Never forget that management, however inept, always has the upper hand.