Enroute Controller issued a 'cruise' clearance to an IFR aircraft inbound to an airport with an instrument approach procedure; noting his belief that the clearance was improper without the issuance of crossing restrictions.

Narrative

A C210 was level at 11;100 FT for the MIA in the area west of his destination. Nearing a point where pilots tend to request lower for approach to the airport; the pilot requested a cruise clearance and I granted it; saying 'cruise 11;100 FT.' Not long after his read back; I recalled that; due to his destination having a published instrument approach; a cruise clearance without some form of crossing restriction was illegal. In retrospect; I feel like I let him talk me in to issuing a cruise clearance before I took all factors in to account. Cruise clearances are increasingly archaic in this era of increasing RNAV approach capability. This was the first opportunity I had to issue one in quite some time; and I applied it incorrectly. Some sort of refresher and official national interpretation of what a cruise clearance is; along with its issuance requirements and consequences would be of use; particularly emphasizing their use on unpublished routes. There was a D-Side trainee next to me with OJTI in progress; and after realizing what I did and discussing it a bit with them; the trainer had the trainee look up cruise clearances in the .65 on ERIDS while proceeding to give; what I interpret to be a bad example; of a cruise clearance off the ground.