Xeroc if you access x.makerdao.com you will see that there is only eth:makerdao market......mpa....open...etc is not listed......
If orderbooks are shared that pretty good.....if we could dvelop togheter better yet.......
The markerdao guys just made a decision to show only their own assets in their version of the web wallet, IIRC.
There's actually been a lot of people who have suggested the same for OpenLedger UI, although I'm not personally in favor of the idea. Personally I think it makes sense for an exchange to "highlight" the markets it is associated with or would like to promote (for example a pair between two exchange such as OL to Transwiser, for instance), but still leave other markets viewable (except to block out any well-known scammy UIAs like the "lisk" one that doesn't honor it's backing).
There's been a lot of concern expressed about liquidity associated with having a lot of pairs, but with bots like ar3b1tr around, you can place orders on markets with no orders and get fills, which I think is actually pretty cool.