I like the idea of having the healer casting riptide on every cooldown and cast dps at all other times. Is there a nice macro for that?
As discussed here, the best bet is to have two separate macros on two separate keybindings - one for when the healer actually needs to heal, and one for when he's DPSing.

There's no way to reliably interweave the roles with a single macro because the macro can't make decisions about when to heal.