I was always led to believe and in fact always played that you could use FW in normal games with your opponent's permission but that this was something of a gentlemen's agreement - you should always ask, and they should always say yes. This was a fair enough system because if someone said no they were usually a prick and one of those pedantic sticklers who are the worst type of person to play against - they would always point to the fact that FW isn't allowed at official events and therefore wasn't legal outside of IA campaigns.
Now it seems to be that you simply need to tell your opponent what the rules are BEFORE the match. This is fine too so I'm really not sure what the fuss is about. Maybe it's an exclusivity thing as FW is expensive, maybe it's seen as 'the rich get richer' - we can buy the fancy toys. Yes, maybe but those fancy toys have an associated points cost so there is a trade off. It's about as balanced as you can get in an evolving game system.
Honestly, I don't know what the fuss is about, clarity is here now, isn't it? From GW point of view maybe it's about pushing FW stuff. I'm often hesitant to buy things outside of the Horus Heresy series because I don't want to have that awkward moment where an opponent says "No." Now that problem has gone! So will I blaze £300 on some of the flashy Eldar toys? Quite possibly...if I don't spend it on Fire Raptor's instead.