Well there's that too isn't there? In an emergency you don't have the 5 minutes it takes to fit your hand down there and fiddle around.
I've ordered the extenders, if we find that we don't think they are all that safe we will probably look at another car. The lady in the shop said it was because the extended belt buckle was coming across and hitting the tummies in accidents. I don't see how as they would just sit under the arm rest of my kids big seats.