Why is this necessary?  Isn't it possible to have just IP addresses in
  hosts.lpd?  Or IP ranges and "partial" IP addresses like '192.168.', as
  is accepted by hosts.allow/hosts.deny?

It's not necessary, it's historical.  Support for numeric addresses
(and v6) would probably be viewed positively.  Perhaps though just
using libwrap would make more sense.

-- 
        Greg Troxel <gdt@ir.bbn.com>