I'll be the devil's advocate for a different school of thought.
Suppose for performance reasons, your vendor recommends that the OS partition isn't "sparse" and wants you to allocate the full OS partition upfront. This results in 10Gb to 20Gb (or more) of unused space on the SAN drive.
This is fine for a single VM, but chances are you will have several "performance critical" servers, each with its own 10 to 20Gb of whitespace overhead. In our environment this whitespace accounted for 20% of our SAN disk. Keep in mind that there are limits to which we should fill a SAN disk (but that is another story).
Management's had a choice
1) Absorb the 20% wasted space on the SAN, which is in addition to other requirements of "white space", and isolate any "full disk" scenario that might occur
2) Put everything on the C:\ drive and risk the drive filling up due to application logs.
What did they do?
Considering that Windows 2008R2 can dynamically expand the host OS's C:\ drive, and can expand the drive when full, management took the cost "savings" and reinvested it in monitoring tools like SCOM.
Now we are getting more than just simple protection of a C:\ drive filling, but we have a more complete systems monitoring in place to address other concerns before it happens.