"Is it essential for a developer / designer to be able to use a Mac?"
Not necessarily, but it helps. Most design companies and graphic designers just prefer to use Macs because they are easy to use and acknowledge the aesthetics of the Mac design.
Students and designers are introduced to Macs at college and university, so tend to stick with them when they leave.
You normally find Macs are also preferred in the music industry and in the recording studios. Newspapers and magazine companies tend to use both. The software they use determines which hardware they use. This is also the trend with animators and games developers.
All office work is mainly PC based i.e. word processing, emails etc . . .
People who play games also buy PCs but I would rather buy a games console. That's what the PlayStation and XBox were invented for.
It's usually the budget of the company that determines whether to purchase a Mac or a PC. If money is no object, then they tend to buy Macs. With Apple switching all their Macs to Intel chips, this will eventually help bring down the prices. I hope.
My personal preference, from experience, is always buy an Apple Mac. I wouldn't give a PC the time of day or my money. I find using a PC (Windows) adds to stress levels.
The PC versus Mac debate will always be around but at the end of the day, it all depends on which you prefer to use, what you can afford and what you want to achieve and output from your computer.