Do i get this all right?
Then
In real no exchange takes place now. Protocol has escrow on Alice’s XCP. Protocol cannot escrow Bob’s BTC so it sends message “Bob if you send Alice 100*0.004=0.4 BTC before block Y you will get her 100 XCP”
Thereby
Maybe some wallet will do this soon? I am afraid of Polo 
Basically, yes.
Because many Bobs used to think it’s all automatic and that the DEx can take 0.4 BTC from their address after they log out, matched orders used to expire because Bobs would end their Counterwallet session and forget. Then Bobs complained and developers removed anything involving BTC from Counterwallet.
You can still do this kind of trade on the Counterparty platform, but just not in Counterwallet (you’d have to use the API or CLI, both of which are bothersome). I think Indiesquare Wallet allows you to use the DEx and you can use the same pass phrase as for your Counterwallet (if you wish).
Poloniex has been problematic because of a suspect Counterparty bug as I mentioned in the other topic.
DEx is great if you need best reliability and can find someone interested to trade (because due to the lack of “BTC Sell” functionality in Counterwallet people can’t sell or buy BTC so BTC liquidity is very low to non-existent) and preferably Indiesquare or technical user who wants to buy or sell a non-trivial amount of XCP (say 100 or more).
I was actually one of those who suggested disabling BTC on the DEX. It was both annoying and confusing with those Bobs who didn’t make the BTC transfer (no funds lost, but still …)
For new users this would put the DEX in a bad light. With pure XCP/asset trading the DEX experience is much better (slow but everyone is aware of this).
With the right user interface (warnings to users) BTC on DEX can work though. E.g. Alice and Bob agrees on the chat to trade 1000 XCP. Neither trust each other, nor Poloniex, and on the DEX they can safely do their trade.
I know it was annoying to some, but we could have simply disabled it (default) and allow advanced users to enable it in Settings.
Soon we may need to introduce it again, especially if we want help Smart Contract users be able to obtain XCP on the DEx.
Definitely, it will have to make a come back soon. I think this announcement will have some serious implications for BTC-XCP trading -
Ruben has created a demo project to
compose P2SH transactions, for testing purposes. Impressively, it also
contains the basic components to demonstrate an atomic swap between BTC
and XCP! This is an incredibly exciting development that will have more
news forthcoming soon. Check out the project here.