Server software, even if it is free, is a HUGE investment. When you spend hundreds of thousands of man-hours on developing web sites and services, the availability of that server software becomes absolutely critical.
1) As other software and as hardware changes over time, the server must be something that can be maintained in relation to what you've developed. If the source is open, you can take it on yourself for maintenance, even if those responsible for it go belly up. In other words, you have a GUARANTEED future availability.
2) If your needs grow to something slightly outside of the abilities of that server, you need to have the ability to implement your needs without having to do something radical, like replacing a 100k man hours of infrastructure development.
Apache, and other open source servers guarantee a certain level of FUTURE-PROOFEDNESS. As long as the source code for your current version is available, you can deal with it forever.
Closed source server software is absolutely out of your hands to control, and for this reason, is NOT a viable investment, even if the entry cost seems to be very low.
With closed source dependencies in your infrastructure, you could be very easily fucked in the future.
I will say this; anyone serving any more than their own person photo album with this software is an absolute and total RETARD.