Survival of E. coli on agar plate at -20 degree C without glycerol stock
Over time I suspect that ice crystals will form in the colonies from defrost cycles in the freezer and the moisture in the agar.  This will cause rupture of the bacteria membranes and over time may destroy the bulk of the bacteria cells.  The plates will also desiccate over time as well from the defrosting cycles of the freezer.  You may be able to leave them in the freezer for a while but I can't foresee them surviving indefinitely.  

If the strains are really important I recommend just sucking it up and spending the small amount of time necessary to make the glycerol stock.  Glycerol is used in bacteria stocks as a cryoprotectant to prevent the crystallization of the stock.