The power of docs to heal and mitigate damage and the usefulness (or lack thereof) of some professions in a raid situation are 2 completely different problems. Changing docs effectiveness in pvm will do absolutely nothing to make people want an MP, fixer, or agent in their pande or (insert raid instance here) team. Nerfing a doc (or a crat or an enfo) in pvm will change one thing - the number of docs (or crats) required to complete a raid.
If the problem is that too many professions are useless in raid situations, then the obvious solution is to give them tools that make them useful, viable alternatives to the currently "required" professions and not to nerf the professions that are currently required. Some possible examples
Means has said more than once that crats are and will continue to be important additions to raid teams because of their ability to mitigate damage (init debuffs, boss calms, etc)
Finding a creative way to give some of the currently "useless" professions weaker (but still useful) versions of some of these tools might make them viable alternatives to a crat when none can be found to do a raid. We currently see this with keepers/fixers. 12 man teams would rather have a keeper, but in a pinch a fixer will get the job done, but not quite as easily as a keeper.
Giving agents access to the SL doc heals up the level 214 or possibly 216 heal. In this way, if for some reason you couldn't locate a top notch doc for a raid, an agent or two, or an agent and a weaker doc would work well enough. We currently see this in the vortexx instance - if you can't find a doc, a couple of advs in concert with a crat will get the job done just fine, although not quite as easily as a doc can.
Of course, the dark side here is that by giving other professions access to these tools, there is the potential for drastically OPing other professions in pvp. I'm sure that with some creativity, however, that such problems can be miminized or avoided, especially considering that means seems committed to removing the ridiculous OPing of any one profession in the long term.
and I'm not saying dont nerf a docs pvm ability EVER...im just saying that changing a doc's pvp effectiveness isn't appropriate until you solve the problem that necessitates their current level PVM of effectiveness.