Raiders complain they have to raid LFR to get the improvements.
They don't, really. The set bonuses are there in normal and heroic as well, and the item level is even higher.
Actually, in Mists of Pandaria, each new LFR tier will have an increased minimum ilevel requirement, so there'll be a natural progression path as the expansion goes on. This means that players will start running Heroic dungeons in order to gear up to hit the minimum ilevel for the first tier of LFR, then get gear from this first LFR tier to access the second, and so on.
However, it won't be mandatory to go through the LFR system if you're raiding normal or Heroic raids.
And since you'll probably ask, I'll answer pre-emptively, at this moment the developers are not ready to talk about possible 5-man Heroic dungeons with higher requirements that might come in future Mists of Pandaria patches.