They're considered "poor grammar" for historical reasons (mainly because they're impossible in Latin), not because they are not understood (compare to "I the do

They're considered "poor grammar" for historical reasons (mainly because they're impossible in Latin), not because they are not understood (compare to "I the dog kick" which is ungrammatical in a much more rigorous sense). From a linguistic standpoint what's interesting is what is understood and what isn't, not what is the most proper.