Why pay for UNIX?

I'm fairly new to Unicies. With the advent of GNU/Linux and BSDs, what are the reasons for companies to prefer AIX, Solaris and other commercial systems?

Solution 1:

Well some software is specifically written for AIX/Solaris etc. while some 'money men' don't trust 'free' software (I've witnessed this myself, someone told me I HAD to spend money on OS!). But most of the time it's to get 24/365 support.

Solution 2:

Because then you have a big name behind it that you can talk to for providing a SLA.

Solution 3:

Because you don't want to get locked into an open system.

-- Unknown IBM executive, 1991