If the plane is half full, tough nuts I am afraid, no refund will be given per their T&Cs. A risk perhaps but you could lower the odds of this happening slightly.. seat plus bookings are taken 72 hours before departure, a quick load check should tell you how busy the cabin is for your flight, then you can make a more informed guess.
Unfortunately the T&Cs do stipulate that seat plus cannot be purchased at the airport... odd but I am sure there is a reason. I cannot recommend going after the twos at the back, when disembarking nearly the entire plane will be before you in the customs and passport check queue - take a very good book if so. ][|) ][|)
As for the question as to whether or not it is worth it.... in my view no, but then again I do like having chats with random people that I am seated next to. I have been lucky with seat mates in the past with only a few exceptions, one a few years back was particularly disturbing :0 but makes that journey all the more interesting.
If space is what you are looking for without the premium price tag, how about the bubble? Rows 75-80 on the upper deck should afford a good space and you get to disembark first too... sorry I have rambled on haven't I
