One of the biggest advantages of using interstem trees is that you can
choose the ideal rootstock, say, 111 for dry soil, and still have an early
bearing dwarf tree. If you planted your trees directly on a dwarf stock it
would not be nearly as drought tough as 111 or as able to deal with neglect.
One of the major disadvantages of growing dwarf trees is their need for
extra care whereas standards and 111 are very tolerant of neglect and can
be planted out on the back 40 and still fend off the elements of weeds and
drought. Interstems can also be planted in such conditions and I suspect,
survive even better under some circumstances than standards since they
don't have as much of a canopy to feed as a standard.