RoF issue is not shards but that it takes a long time to do damage, so all the packs that last like 5 seconds get stomped completely before RoF can do damage, another issue is that things need to stay inside it as well. And herein lies the problem - many classes/specs have absolutely stupid burst AoE, so things get roflstomped way before you have opportunity to do real damage to them.
Cataclysm does help though, but it has a cost to it.
As for RoF - 3 Shards is OK cost for RoF, the thing does good damage actually - it just has those issues I mentioned. If you actually able to utilize RoF 100% and get to stack it - it's one of the best baseline AoEs there.