This feature to execute startup script sounds promising.
So finally we have some replacement for MSBASIC to run startup commands without need for macros?
Explanation is not very clear as it mentions installation but if it will allow to attach some cell library as default using script each time program is started then it is great.
-S switch has always been available, even in V8i. I think the new user prefs option is to provide a means to ignore the switch should a user wants to. As to why Bentley think that's a good option to have at user level, I can only guess. Command line switches are mainly used by CAD admins to get the right resources, and now users have the option to bypass those switches.
There's actually a defect filed against -S switch, defect 1039704 (filed against Open Building Designer actually).
In V8i, the command file will be run regardless a dgn is specified or not, i.e. if a user manually select a dgn file after startup, the command file will still run.
In CE, the command file would only be run if a dgn is explicit specified in command line. Opening a file from backpage will ignore the -s switch.
So a technology preview feature developed for a currently defective feature. It would have been much better if Bentley spends more time investigating this idea: