Same as others on here - I have my middle name on tickets maybe less than 50% of the time. Never been an issue (touch wood!)
IIRC there was some story in the news recently about a low cost carrier from the states (WOW Air or something?) that refused a whole bunch of poeple travel because their middle names weren't on their tickets. But the theory behind that is that they were overbooked and just looking for any excuse to deny travel.