The reasoning about segment cost effectiveness is as follows:
1) You are permitted a maximum of 20 segments on any ONE ticket.
2) With a 3 continent ticket you have only 17 segments (3-IC, 4-EU, 6-NA, 4-AS). Without the 20 segment max, a 4 continent ticket would have 22 segments, a 5 continent ticket would have 27 segments, while a 6 continent ticket would have 32 segments. Thus with the 20 segment maximum, in a sense you are paying for, but unable to use, 2, 7 and 12 segments respectively.
3) Using the current AONE prices, ex-DEL ($8890, $10080, $11592, $13331), then the price per allowed segment for the AONE3 is $523, for the AONE4 is $504, for the AONE5 is $580, and for the AONE6 is $667. Thus the AONE4 is the most cost effective in terms of cost per allowable segment.
4) It is true that with an AONE5 and an AONE6 you get an extra intercontinental segment on each (and it may even be stretched a little further with creative routing) along with more flexibility of continents to which you can travel. So your average segment length might be a little longer and the cost per mile traveled might be narrowed somewhat. Because this difference depends on actual routing, one can’t say in general what these figures would be. For this reason, an AONE6 is not obviously a bad deal, but it does take some analysis to make sure these advantages outweigh “throwing away” the 12 segments that you can’t fly.