Opinion is, healers shouldn't be forced to DPS. What's expected from them is that they keep the party alive, not that they DPS. A lot of people here mix the optimisation of a role with the requirements. And it's not comparable a DPS who stands in the fire with a healer that doesn't DPS. Dead DPS can't add damage which is what they should do, healers that doesn't DPS can keep their party alive perfectly. If anything, you could compare a healer with a NIN who doesn't Goad or a Monk who doesn't cast Mantra. Does it ease the burden of your companions? Yes. But your party won't wipe from that.

There's also the matter of laziness. First of all, except for really obvious cases (but, like, really, really, obvious) you can't just tell if it's laziness or just that the healers aren't comfortable with CS. And, in DF, you'll have scarcely a moment to heal if, let's say, you get a party full of bozos who move at Scathach shadow ground or get caught in the Shadowspins. It's really full of circumstances and the time a healer won't DPS out of laziness are fewer than some of you make it look. Also, not everyone has grinded the instances to the point of being able to know when each time happens and not everyone is in a static group where they know they play with people that are familiar with mechanics and timers.

On the 50% tanks matter, I'd plainly say that leaving them this way is just unnecessarily incresing the risk of the tank dying due to a tankbuster out of nowhere. Topple them up isn't that hard and doesn't have to take a huge toll on the DPS. It's better to be safe than to be sorry.

At last, I'd say that, everyone should try to do the best they can as long as they feel comfortable. It's no point to toggle CS on and off if you can't anticipate when will the heals be or not be needed. But if you think you can do it without a problem and you don't find yourself under too much pressure, go for it. It's not something required from healers but from players as part of a team. Everyone should try to fulfill their role and also try to help the others in it when they find themselves able to do it. Fulfilling the job's role is fine, but reinforcing the other roles' performance while at it is superb. That's what teamwork is about.