Still, I think Unimatrix Q has a point. Yes, alien cultures should have diversity of naming habits as a general rule, but in this specific case, it makes sense. Since DS9 established the pattern that a joined Trill's two names represent the host and symbiont respectively, and since TNG's version was all symbiont and no host as far as identity/personality was concerned, it made sense that they'd only use one name in that case.
After all, given what DS9 revealed about Trill joining, the joined are a fairly small percentage of the population and they're all approved by the same body. So it stands to reason that, even if there are multiple cultures among unjoined Trill, the joined Trill represent a single subculture with standardized practices.