Lets list the ways in which this is a bad idea
1) Cost of hardware. Macs cost significantly more than a PC
Ican buy a Dell for £300. It's nothing special but it will do everything %90 of office workers need to do. Macs cost significantly more. And lets not even talk about support.
2) Cost of support. Supporting multiple OSs, PC types, etc is more expensive
How many people out there have experience running Macs against a windows domain? Go on, raise your hands. Thought so. Finding people who can do this in a credible way that doesn't expose the network to security issues is going to be expensive. Windows support people are plentiful and cheap.
3) Cost of IT time to make business software run on Macs
I've read lots of "ways to get windows software to run" on a mac. All of these require effort, setup and support. There's a massive added cost here in getting people to a position where they can do their job - as well as wasting ITs time helping people getting software to work on incompatible platforms.
Apple has a long history of leaving security holes to linger in their OS before patching them. Their recent issues with the Java runtime was a case in point.
Companies are there to make money. IT is there to help them do this. Giving people equipment that doesn't allow them to do their job and costs significantly more to run and support is idiotic. I completely accept that there are places for Macs in certain roles: graphics design, some creative functions, etc, but if someone says they want to use a Mac to use Word, surf and run the accounting app then they need to be shown the door (as do the idiots pandering to them).