As someone who frequently plays SCH, and DPSs quite a bit, I can see both sides of this argument. Personally, I'm of the opinion though that you should DPS when you can, but never to the detriment of your healing. The problem is that the community values speed above all else, so they forget that a healer DPSing constantly means they're not healing. I'd always take a healer who never DPSs but keeps the party at full health over a healer that DPSs constantly and speeds up the run, but lets half the party nearly die before blowing all their cooldowns at once to bring everyone back up. I have been guilty of falling into the latter before, of course, but typically only in content I've healed plenty before and after I know the tank can handle the pulls they're doing long enough for me to get some spells off. If I'm in content I'm not as familiar with, or if I know that the tank is undergeared or bad, or that the boss is going to be doing heavy damage, you can bet that I'll stay out of Cleric Stance and stand there preparing to keep everyone healed up. If someone isn't comfortable DPSing as a non-DPS role, they shouldn't feel forced by their party members to do so......
Unfortunately, the mentality of the playerbase isn't going to change anytime soon, simply because there are less and less players who are playing for the content itself, and more and more are just playing for the "end goal" that you get when you beat the content. As a result, many just want to clear it quickly, even if it's to the detriment of both their experience in the run itself, and that of the party. To add to this, it seems like fewer and fewer players are actually willing to listen to the advice of others should they be doing something wrong in their role. This in turn means rather than the poorer or newer players improving, they're having to be carried, which in turn forces more Healer and Tank DPSing to be required rather than optional. Of course, it isn't just those players faults, since fewer players are willing to explain mechanics of bosses even if they're asked simply because a high DPS party can ignore the mechanics, which hurts the newer players when they get a lower DPS party and have no idea what to do to keep from wiping. I've been playing for two and a half years, and there's some runs that I still have no clue on the exact details on the mechanics on certain bosses......