My /(ba)?sh/-fu is notoriously weak, so I referred to the bash manual:

 3.5.3 Shell Parameter Expansion

${parameter:+word}
    If parameter is null or unset, nothing is substituted,
    otherwise the expansion of word is substituted.
[download]

I take that to mean that if argument $1 (ie. ${1}) is null, nothing gets substituted, but if not, it's replaced by all arguments.  In other words, perhaps it's a clean way of referring to all arguments, and not failing if none are specified?  That's my best guess, anyway.