You are right to some extent. Consensus node can do all it needs for Steem platform to run. That's steemd running with "low memory" settings, keeping in memory only things that are absolutely needed for Steem platform to work (that's getting newly produced blocks from the network, broadcasting transactions etc.). If you are looking for more, like the ability to respond to certain API calls you can add plugins for tags, follows, account history. For some calls steemd with "low memory" is ok, but to be able to use all the API calls you need to connect to full node that has everything prepared to serve your requests.