In my 8 year airline career, I learned SABRE (took a 6 week class in the 90s), Apollo, and DLMatic and also various airlines' own operational systems for flight planning, weather, times, weight/balance, etc. When I became a duty manager in SOC, I got access to pretty much all operational features. On of the more interesting features I remember learning about was SABRE's "FIT" function which stood for "Flight in Trouble". That was designed to lockout a flight that is in trouble (i.e. crash) so that no one (but admins/managers) can see the manifest or any other info about the flight. Thankfully I never had to use it. I still have a SABRE manual somewhere in my boxes of airline stuff.
Out of all the ones I learned, I liked SABRE the most and DLMatic the least. I also used DLMatic the least (and learned it last), so that probably is why it seemed so foreign to me compared to the others. I still remember how to long sell a flight in SABRE and I think it was the same in Apollo: 0DL2633YRDUATLNN1 DLMatic I think was similar, but maybe they used an N instead of a 0 at the beginning.
0 - Sell
DL2633 - Flight
Y - Class of service
RDUATL - City pair
NN - Need Need
1 - Number of seats
Then you could use WPNCB (Will you price nice and cheap and book) to have it find the lowest fare in that class.
I once sold a last second flight to a guy in 30 seconds from the CLI.
I'll take the CLI any day. The UI is all built on the CLI anyway so it will never have as much functionality (because features are added all the time). I remember when I worked for UA my supervisor threatened to write me up once for using the Apollo CLI and not using the crappy UI that UA wanted agents to use. Later that same day she came to me and asked me to help with a complicated ticketing transaction. I showed her how to do it in the Apollo CLI.
I've considered getting a SABRE subscription (without ticketing authority) just to be able to quickly peruse availability. I wish there was some way to download the whole OAG schedule and I'd just build an application around it, but every option I've investigated seems pretty expensive.