It depends on what kind of cotton your comforter is made of -- The cotton comforters with polyester lining I grew up with had fairly soft outer parts which were much like a sheet so washing certainly didn't have adverse consequences and they were thrown into the wash regularly.
However, one can't wash chintz or other cotton fabrics with a "stiff" or shiny finish as the finish would be gone. Some cotton prints might bleed or fade.
So it really depends on whether your comforters are more like bed linen or more like fancy bed upholstery -- if that makes sense.
A lot of stuff says dry clean only because that is the safest thing for a manufacturer to say even if a product can be washed appropriately -- silk blouses or wool sweaters can be washed (and it's actually better for them to be washed) but the instructions almost always say dry clean only.
Some days I pray for Silence, Some days I pray for Soul,
Some days I just pray to the God of Sex and Drums and Rock 'N' Roll.