CMs and the lower APs are blocked on certain days because Disney expects large numbers of day crowds, e.g. Christmas Week. DL hits capacity 2-3 times on average that week each year with CMs and most APs blocked. (APs usually make up only 20% of the crowds during that time -- they're not the reason for the crowds then.) There are crowds on those days, but the make up is different. Being in crowds of mostly newbies who don't know where anything is located, don't understand the FP system -- or even know that it exists, don't know where the restrooms are, etc. is a different experience from being in crowds of experienced park goers who know the ropes, know where they're going and how to get there quickly, have the app and know how to use it, etc. The parks are just crowded anyway -- it's just a different kind of crowded depending on when you go.
These days, I think this is a practical and sane way to look at it.
Some people choose dates based on hours, some based on weather, some based on events in the parks, some based on when it's possible for them to go, etc. Go when you go -- just have a sound plan, a backup plan, lots of patience and a positive attitude. Then get ready to have fun in the parks!
