Fewer build options = easier to test/ship/support ICU (would remove one little release task)
Using ICU in a single-threaded environment is ''slightly'' more efficient if ICU never attempts to lock mutexes etc.
It is easier to port ICU4C to a new platform if you need not support threading, but even cell phones have multithreaded OSes now...
Simplify mutex code
Users could use existing API to override mutex functions with dummies
Document workaround via dummy thread functions
Take ICU_USE_THREADS out of uconfig.h but keep it in the test code