>Small office, Win+Mac clients plus Linux backend sounds like either too many cooks or too much money ;-)

Not necessarily so - sometimes two businesses, no matter how small, merge for other reasons, not considering the compatibility of their software. And then whoever is in charge of their IT has to put these together somehow, which is usually done in small steps, gradually, unless one of the systems is fully capable of covering the whole new branch of business, which almost never happens. So the two or more systems coexist for a few years and you get incredible mixed environments.