Take HS for example, as it's always booked up. If you didn't have to visit your 1st park, people would simply sign up for Epcot and go to HS asap. So far, hoppers have never been denied entrance. I suspect if they removed this requirement, we'd start seeing hoppers denied entrance due to capacity limits. The combination of waiting until after 2 and having to spend time at that 1st park is preventing this.
When people ask why Disney isn't selling APs yet, this thread is the largest example as to why. It can be frustrating as a local AP holder (I'm not local, but I get it) to have this AP and be told you can only reserve 3 days at a time. With park capacity filling up, the value of that AP is lower, because you aren't guaranteed to get into a park even 4 days in a row. I do agree, for any AP holder, the inability to do anything more last minute can be frustrating. I'm with those hoping as capacity increases it gets easier.