I think we've covered the OP's questions about the problems with wireless cameras, and the dealer's preference for wired systems.
The dealer used a simple(even if it is incorrect) description of the problems with inexpensive wireless cameras, so as to promote wired cameras, which he feels more comfortable selling to a customer.
I think many people(me included) like to have as much info as they can regarding technical problems with electronic devices, so any discussion that adds to that knowledge base is welcome. Ground plane info was unknown to me before this thread, and I found it very interesting.
If extra technical info is not wanted then it would be appropriate to just allow ones eyes to glaze over and say "uh-huh".