Yes, seating the Michigan and Florida delegates is critical, because the voters of those states should feel included. It is not the fault of the average voter in Michigan or Florida that their party leaders changed primary dates. The voters would feel like they did not participate in the election if the delegates were not seated.
Seating the Michigan and Florida delegates in the 2008 United States presidential elections is not necessary to preserve democracy. They did some thing wrong to be stripped from their delegate status, so seating them even after they were barred from being a delegate would send the wrong message to the party base.