The name is so you can have multiple independent locks. Eg, you might be using the lock you describe, and you might also want a backup script to run every hour, but if the backup took more than an hour, you would not want two backups to run at once, so you could use a lock to avoid that, and use a different name so as not to stop your other macro from waiting for the backup to finish before executing.
It would certainly have been a valid design decision for every macro to run in sequence, but that has never been how Keyboard Maestro operates. Keyboard Maestro can have many different macros all executing "simultaneously" (they actually don't execute simultaneously, its more like tiny pieces of each macro execute in succession).
If every command executes immediately, then the sequences would be in lock step. So imagine a case where two macros are executed in relatively quick succession, but each macro consists solely of 10 Set Variable to Text actions (which never have to wait for anything). The sequence might be something like:
A1, A2, A3, A4, B1, A5, B2, A6, B3, A7, B4, A8, B5, A9, B6, A10, B7, B8, B9, B10
But many actions have to wait for something (roughly 40% of all actions do not complete immediately). Even actions like typing a key wait a period of time, and that time may not be precisely consistent. Once that happens, the second macro may catch up or pass the first macro depending on the various timings of its actions.
It would be a perfectly valid design decision. It's just not how Keyboard Maestro works, and I would not want to add a preference that completely changes how macros are executed.
Sure, but 9 times out of 10 when you go and add that macro you would long since have forgotten you toggled some obscure preference and wonder why all your macros stop working.
I don't know of any particularly good article on this. Hopefully one day the wiki page on the action will include lots of good information, but it does not even exist yet.
For the same reason that there is no such preference, and that you would be happy to enable the preference, it really is an action that is rarely needed - most macros are independent of all other macros, and most macros run very quickly, and rarely do you want to do anything else while doing them, so the action is not often required and thus not well known or documented.