From an openmcl-devel thread on Unicode support strategies, specifically on the subject of internal representations of strings, Gary Byers writes

I do think that the point that 16 bits is probably enough is a good one; the subset of UTF-16 that doesn't include surrogate pairs - which I guess is often referred to as UCS-2 - is probably a more reasonable step (if a 2X increase in string size is “quitting cold turkey”, a 4X increase is probably “quitting cold turkey and then getting religion”, which does seem a little drastic ...)