[YSFEJ4SWJUVU6]

> to avoid things exploding because a zero length variable might exist is a stupid and ugly hack.

That has less to do with POSIX compliance, as opposed to working around quirks with less-compliant historical implementations. Also, I think you'd have to dig pretty deep to find a test that doesn't support zero-length arguments; you'd be safe as long as you quote your variables.

The actual problem of ambiguity arises from variables that are also syntax, like '(', or '!', when using compound expressions (which you should avoid anyway).

  # this is bad (also obsolete)
  [ "$var" != foo -a "$var" != bar ]
  # this is good (and fully POSIX compliant)
  [ "$var" != foo ] && [ "$var" != bar ]
  # you probably don't need to care for systems where this was necessary
  [ "X$var" != Xfoo ] && [ "X$var" != Xbar ]