The main issue with sapphire displays would be its yield and cost, as making sapphire for the camera lens is going to be far easier than to churn out an entire display for a handset. Apple is said to roll out approximately 10 million of those 5.5″ phablets, where a single sapphire screen of such size has been rumored to cost somewhere in the region of $280, which would mean the entire handset is going to cost a whole lot in the end.
There does seem to be an issue with the yield of these sapphire displays at this point in time, which means the rumored Apple phablet (the iPhone 6, perhaps?) might still be some ways off from hitting the market.