You could pull CAT-5/6 if you have a reason to. If you are looking to put in-ceiling speakers in and do some higher-quality distributed audio system, then that would be a great way to do it with a bunch of single gang control keypads (or double gang).
It really depends on what you want to do, what your budget is, what the possiblity is of adding ceiling/wall/shelf speakers to the rooms w/wiring etc. I (personally) like distributed audio systems if done correctly, but would probably drywall everything up, pull out all the wires that weren't being used, and start anew. But, as I said, it really depends on what wires are there already and what the ability to add new wires & speakers is.
Can you believe that the MODERN day intercom systems look and sound almost identical to what you have from the 70's? Think 70's is exactly what I thought when I walked into the model home in the neighborhood I bought in. Thankfully, they can run my system instead of theirs.