Landing at Honolulu the crew was told "Next available, Ground point 9 when off". To me, sitting on the jump seat, that was blindingly obvious; take the next exit, then call Hono' Ground for taxying instructions on the VHF ground frequency which ends in .9. But no, the crew blundered off onto the next exit, then turned straight onto the taxiway without calling Ground, causing a 747 to come to rather a rapid halt, then struck up a conversation with the busy local controller...
Some 'slang' terms and phrases may very well be better than the current ICAO language, but the point is unless they are standardized, some people may not understand them, as the above example shows. Had standard phraseology been used, the crew in the example above would have known exactly what to do and which frequency to call. As it was, they weren't told whether to take a left or right exit, (I am unfamiliar with Honolulu), and they weren't told to vacate then
hold position while contacting ground.
So what might be 'blindingly obvious' to one person might be totally confusing to another, and it is the latter that will cause an incident or an accident one day. That's really the point, I think.
Another point is if controllers are having to resort to their own slang and verbal shortcuts because they are so busy -
then they are too busy!. They need to file a report of some kind to get more controllers for that sector or whatever needs to happen. Doing their own thing with their own invented phrases might actually be dangerous, and the one occasion when someone doesn't understand a non-standard phrase and taxis into the aircraft taking off or whatever, simply doesn't bear thinking about.