RDM/* should always act as a backup healer, IMHO. In a pinch and at lower levels (less than 30), a RDM/BLM or RDM/WHM *can* act as a main healer given several conditions:
1) You're Tarutaru
2) Your tank is a PLD
3) There's a backup healer
4) You're chaining T / VT
5) You're fighting near zone ;)
Pick two.
The deal is that as BLM, you get a good INT boost - your MND is lower, but it doesn't affect your healing that much because the Cures cap out early enough that it doesn't make a huge difference. But you're still RDM main, and even if you're trying to press the point that you're specializing in nuking, it ain't a perfect world and groups can get by without a nuker, but not without a healer. When things get desperate, they'll grab the RDM/WHM.
What a RDM/BLM misses most in that department is poisona, protectra, shellra.
What a RDM/WHM misses most is area-effect nukes. Of course, AoE nukes are dangerous, so maybe that's all for the better.
All in all, as RDM main, no matter what you sub (no matter WHAT you sub), your role in the group will always be decided by the group's makeup. You could be RDM/DRG if you really wanted to, and you'd still be asked to heal if the group needs it, still be asked to nuke if the group needs it. So figure out what appeals to you more, very rarely using a Stonega and having Warp, or very often casting Protectra and Shellra to save the WHM's MP, and dealing with status effects in combat while the WHM is chain-healing.