Print Driver Isolation: What exactly constitues a printer driver instability, failure or bad driver?

Server OS: Windows 2008 R2 Standard

The Microsoft PDI blogs and HP PDI Documentation detail PDI prevents a printer driver instability, failure or bad driver problem.

What exactly constitues a printer driver instability, failure or bad driver? They're rather vague terms. Is it specifically poorly coded drivers, drivers causing problems that are "by design" etc...

Could someone please explain how PDI knows this?

OR

Is it simply a case of using PDI to proxy off print jobs to an isolated host process keeps it separate from the main Spool process and therefore the theory being drivers that cause a problem (any problem?) cannot interfere with the main Spool process?

Thanks

Alex