You're basically right though, there are regular calls on /r/bitcoin for people to run a full node because the number has gone down recently. I decided set one up on my dedicated host due to one of these.

Interesting - I have been meaning to look into how the blockchain works (I missed the get-rich-quick rush and dismissed it all until heard Andreessen explain the reasons)

Anyway - what does being a full node entail?

Simply run bitcoin-qt or bitcoind all the time and make sure that you have port 8333 open to the outside world (if you have a router, this usually means forwarding 8333).

It takes a few hours to a half day for the node to catch up, depending on your bandwidth and CPU, but after that it requires relatively little processing time. But you do need ~20 GB of free hard drive space for the blockchain.

Non-mining nodes just hold and relay validated blocks, they don't do anything that can be hijacked.

Full nodes host the memory pool for the txid's - So they serve a purpose: Making transactions propagate over the network. And serve blocks to nodes that are not up-to-date (And clients).