I've only played using the demon codex 3 times (I think)
I took 4 icons (thats *a whole squads worth of icon*)
On evaluation I see that:
to hedge your bets you need 4, you need one for each deployment group and then assuming that the one that has the icon will land perfectly (and or run to an opitmal position) is just cocky.
I found that with the new disruption table dieing can be not quite as bad as you feared (tempered by how much terrain you have).
If someones going to give you a bad day (such as a 200 model army that fill their section of the baord behind a mech screen wall) its going to go wrong anyway..
Fitting stuff around an icon is tricky cramped in some cases (bum rush the icon)
Making a survivable icon is tricky (e.g. 8 bloodletters.. if there are two with icons.. thats 16 guys I need to take out to make you worry,that I was going to shoot at anyway (duh

).. thats not that hard).. so plaguebearers and tough stuff (or max horrors with changling) become necessary.
I think some of it is lazy modelling (not here of course

) because Icons are a fiddle to get awesome.
Basically its too much, makes a unit an incredible fire magnet if you don't invest and can be of little help in some games anyway..That said I'd playtest something like this a lot more than I have to take that as gospel.. I haven't had time to read the warseer
tactica demonica yet but it might have some tips (if you find them drag them back here

)
BTW I think they made icons prohibitively expensive specifically to stop people making rational chaos (demons are meant to be random its
RAW- I just wish possesed and dreadnoughts weren't stupid.. stupid isn't chaotic its just stupid)