The problem is people deposit their STEEM or SBD on an exchange. If the exchange's node is down without the user realizing, then the deposit goes into limbo where the exchange isn't aware that there has been a deposit. It won't show up in the users account on the exchange.
Here are examples of what has happened in the past.
Poloniex node went down for 6 months starting at the end of January 2018. A few months back, Binance node went down for one month. HitBTC node went down at the same time as Poloniex and HitBTC has never gotten their node operational.
The problem is serious.
The exchanges could take weeks, months, years, or never get their nodes operational. If the exchange never gets their node operational, then those deposits are forever stuck. The users will never get their currency back.
A pretty good example, what a terrible EVIL are those centralized exchanges. The sooner they disappear - the better for the crypto world.
The problem with decentralized exchanges is the incredibly slow speed at which they can operate and the insanely low volume. Centralized exchanges can be good if they are operated properly.
One way to deal with exchanges that ignore STEEM is just to create an exchange where STEEM is the central trading pair. Check out one of my old posts that explains why it would be good for STEEM https://steemit.com/steemit/@socky/steemit-inc-must-build-an-exchange
Totally agree with your idea, because steem transactions are so fast,and in the exchanges everything goes different, with STEEM and SBD as central trading this would change EVERYTHING, I'm in love of that idea, spread it out, maybe someone like it and came up with it