the chain guides got the top roller way up there to help the suspension.
Here's my reasoning why they do it...............
*Having the pivot down at the BB gives very active suspension (spesh, trek etc)
*Having the pivot up higher/foreward (still in line with the chainrings) (SC, orange, commencal) gives better pedaling (the extreem example being the prototype orange's that MOJO had a year or two back with the S shaped down tube)
*Now a DH bike needs plush suspension, and lots of it. So youd think having it back and low would be good, but that leaves you with a very curved (shorter swingarm = smaller radius) axel path.
*But what if you could move the chainline independantly of BB height?
you could put the pivot way up high,
keep the swingarm sensibly short (keeps it stiff),
the curvature of the axel path is less of an issue (its stil a circle, but its not pulling forewards towards the seat at the end of its stroke)
and move the chain about to give it just the right ammount of chain growth.
Having the pivot that high without the guide whould result in a bike that practicaly locked out under pedaling!
Its not uncommon, Trek used it on the Diesle, Sesion10, Blafa on the BB7, its less an issue on XC/trail bikes as the suspension doesnt have to move as far round the circle.