Unless under very specific circumstances, the use of a modular kernel brings one the ability to replace the particular hardware the system runs on at will.
If I do taylor my kernel for my machine, I do not care at all
Say, it's possible to replace a just burned motherboard with an Intel CPU with a different one having an AMD CPU instead. Or one may take the HDD holding the system and put it into a wholly different box, while often retaining the ability to boot.
Not true in general unless you compile the whole modules set which takes ages. You probably never rebuild the stock debian kernel on a old machine ;-)
For these reasons, in the majority of cases, compiling a non-modular kernel doesn't worth the effort, and may also be harmful to the system's operation.
Well I do this happily since 96. Almost never used the debian kernel more than a few days after install. Crashed a lot of disks, replaced mother boards, changed computers... Nothing that booting knoppix iso cannot fix and a lot of time gained each boot and when compiling a new kernel version...
-- eric