I don't see much difference.
You sign in polo then you click on exchange and you there.
You sign in OL, then you click on the trade tab and you have a list of the markets, you click and you are there.
The second click you have to do is only the very first time you use it. It's to select one market, which is also something you have to do in polo but it is in the same screnn. After going to the DEX for the first time, the trade tab links you directly on the last pair you watched.
On the other hand, it's also a wallet. IMO is better to display something simple on the first screen for everybody (not everyone wants to go to the trading platform, is scary for certain person that just want a wallet). The ones interested in the DEX, will certainly spend the time to click on the "trade" tab.