well.... its not all wrong... but I don't think you're completely right either...
different cable does make a difference... because cable itself also hold resistance... "the load".. I believe its called impedence.... especially for a long wire... it does make a difference...
I mean... if you're trying to make a perfect system...
you don't want to cheap out on cables...
yes, if the plug type is the same.. then you can basically use the same RCA wires and plugs it on to composite connections... you will still see the pictures... but will it be clearer if you use a good cable ??...... I have not tried it myself.... (my tv doesn't have composite...

)
(another thing is a regulated power supply.... makes a world of difference....)
for example... like speaker wires... if you really use the good stuff... like monster cable.... or something that is soooo freaking expensive that you empty your bank account and that thing is not long enough to hang yourself... it does make a difference...
(I know... because I tried.... my friend is a Hi Fi crazy people... and we hooked up different cables.. and I can actually hear the difference... and I'm no expert...)
so... I would say.... not all expensive cables are ripoffs...