The demo network would be for a lot more than just trading also.
What else?
Anything you can think of that BitShares might use or might use BitShares. Testing ground for games, gambling, messaging, chat, data storage, web pages, dns, security mechanisms, social media, smart contracts, scripting, IoT, governance models, voting, predictions, micro-transactions, market Bazaar. You know, all the standard stuff.
what do you mean demo liquidity? If there's no cost to obtaining demo coins, providing liquidity is easy.
We struggle to get general liquidity on the DEX with the maximum number of bitshares users - how we will get anything usable on the demo network with a tiny subset of the userbase?
Because there is no cost, and no risk, it only takes 1 person to provide all the liquidity you would ever need. I wouldn't be surprised to find the demo network more active than the real network, again because there is no cost and no risk. Someone with no money can't use bitshares proper, however they might be curious with a free demo network. It would even be interesting for existing bitshares users just to test out whatever experiments are going on. Especially if there are games and gambling which are taboo (for some reason) on the main network.