Yes, it seems that the purists of GTD do not feel completely at home with the Data Model behind Toodledo.
I have used GTDAgenda which fits 100% to David Allen's model, but their interface is mediocre and they don't support collaboration which is a killer for me.
How about outlining clearly what we miss and ask the Dev@Toddledo to look into it ? Would not be that difficult I think...