I don't think it's cheating at all. I was enjoying the dual box experience with my second account, made the kills much quicker. But it's not really something I'd consider practical for an actual xp party unless you get really good at it. It's easy for me to make an /assist Aksannyi macro but with parties the dynamic is totally different, you might have to pull or tank.
I know people who dual box for farming. They use their thf main to do the killing and sit their other character at the zoneline naked to take all the drops, inventory fills and they go into town and NPC everything, rinse, repeat.
If you get good at it it's great in case you don't have a lot of friends to kill stuff with, sometimes having your dual-boxed character around it can be a huge help with mobs you couldn't otherwise solo.
I'm not a fan of the dual boxed PL, especially when the person doing the PLing isn't really doing anything, though I can see people's reluctance to level without PLs nowadays and it's become much easier for someone to just dual box it than to try to find a willing friend.
Like anything in FFXI, it has its good points and its bad points, but no, I don't consider it cheating. Unless they're RMT dual boxing.
