Not quite true. Any PPO ADR is allowed in upon opening the gates for ADRs. So, an 8:30 ADR will still get you in when the gates open a little before 8 (depending on the park for exact time). However, the later ADR may (but not always) result in you waiting awhile for your table, resulting in not finishing your meal in time for rope drop.
As to rudeness, if you (in good faith) make an ADR for 8, but half decide not to go, then the 4 who want to go should keep the ADR, and won't be penalized, and I don't think this is rude. The problem is Disney does not allow modification of the ADR. Chances are, by the time half the group changes their minds, there will not be an ADR for 4 available. So, keeping the ADR for 8 to allow the 4 to go is not being rude; Disney is not giving them the option of opening up the other table. The 4 who want to go should not be penalized by Disney's lack of ADR modification ability.