if you use a good RCA there is not much difference to BNC. RCA first was designed for coax cables for easy attachment, and when it was popular in RF/HF market in old days, it was only good enough plug/socket to use for phono (there comes the phono name for plug) connections - and there for became popular in audio use.
I personaly don't like it, nor BNC. Both lack of stability (for moving cables both have glich problems) and robustnes - both are not ment for touring abuse, though BNC is popular and a bit better.
If you buld equipment (or even a pach board) your self, check out other connectors like SMA (it is small, and really good contact).
about sending video trough cat-5, much depends form individual devices (never use pasive ones). For those who are in DIY check out LM7121 IC's. it is not that hard to make one yoursef if you have good skils in soldering (it is smd only
).