We don't really deal with smaller enterprise, so I'm not sure how well my experience will relate. We tend to find that our clients treat "our software won't work with your PC" types of problems as OUR problem, not theirs, and something we should address, not them. I'm yet to see a client bring it up - we have to be pretty pro-active, and we've been caught in situations once or twice where we've had to scramble to support older browsers at very short notice, because they were running very old versions and gave us the choice between making it work on their systems or them considering us in breach.
The specific situation that seems to cause it is that site managers are responsible for budget for purchasing IT infrastructure, but central IT manages the infrastructure. You get a few site officers refusing to retire PCs that are well past their expected lifetimes, and central IT says "sure you can keep using it, but forget updates: the latest OS we've tested on that hardware is XP without SPs, and the latest browser we've tested on XP without SPs is IE6, so that's what you stay with." I think central IT are trying to force the site managers to spend budget on IT gear, but IT is often not involved in our proposals (they just manage the infrastructure,) so all that happens is whichever higher-up decided to go with our solution tells us, basically, "I don't care if there's a fight between a site officer and our IT dep't over budget, or if MS have deprecated that technology, or whatever other excuses you have - we've bought your solution for our EXISTING infrastructure, and you need to make it work on that."