You need to use them correctly in order to make them worth your while. It seems like the happy medium is about 100-130 seats for nearly every route I've created. When you go larger than that (without competition on the route) you lower the demand and in turn lower the ticket price.
I was originally putting the 757 on flights between 350, 300 and 250k airports because there is enough demand. That worked out alright until everyone else started putting the same plane on the routes. So I went from a 1.3M DOP per plane down to about 500-600k, which, coincidentally is close to the average I'm making per 737.
Something else to consider which many players don't seem to understand, is that not every route deserves a 727, 757 or 767. Granted, I'll OCCASIONALLY put one of those planes down on a 350k to 50k airport, but it's mainly to use up hours. I've seen several players using these larger planes on pointless routes where THEY ARE THE ONLY airline on the route (until I move in to it of course). Their logic I'm sure is "well, if I make 125,000 on the route now with a 115 seat plane, then if I upgrade to a 230 seater I'll be making 250,000!!!". In actuality (from my experience trying this) you run a 15% chance of making less money on the route, about a 60% chance of making +/- 3% (essentially the same) and a 25% chance of (at best) making 10% more on the route.
On the other hand lets say that on one route you could fly a 737 for $600 a ticket and a 757 for $350. You start with the 737 and over time 6 other airlines move on to the route and you all have to pare down your fares to about $375, essentially cutting your profit down by 40% it's original value. Upgrading to a 757 now would obviously make you a significant amount more on the route.... But you've also screwed up the equilibrium causing the other players to now need to undercut you in order to maintain 100% LF.
And so again the mouse chases the cheese.
Point of this diatribe? Unless you have a hub at a 300 or 350k airport with lots of gates at 250k+ airports, or you have lots of competition, it's not really worth it. But... Since a majority of the players don't realize that and are buying them just for the sake of having one, you may need to buy one in order to keep up with the Joneses.
