Most of the big players in the calendaring & scheduling world have some support for the iCalendar standards (RFC2445 at least, RFC2446/iTIP and RFC2447/iMIP to some degree). Some have better support than others. What all of them are missing, of course, is good interoperation with their competitor's products.
I'm making an educated guess, after three decades in computer work, that the vendors are afraid of true interoperation. The marketing guys reason that if their product is interchangable with someone else's, which would be true if the interoperation parts worked, they don't have a hold on you and you'll just switch out for something cheaper. They're wrong. Some people will switch out for cheaper stuff, but usually that's a fairly small minority. People want software that works for them, and more often than not will continue to use one vendor's software for years if it works well enough. So, the majority of people who switch do it because the other product wasn't getting the job done, or it was doing the job badly, or in some other way interfering with business or personal life.
Now we're reaching a point where a lot of interaction between businesses and people (or other businesses) takes place across interconnected networks. Nearly everyone reading this communicates via e-mail. Everyone uses different e-mail programs, but they all can send messages to each other, because the e-mail programs comply with some standards to make that happen. If you're a computer support person, you know that it's complex and hairy stuff that makes it work, there are bugs and flaws and arcane things going on in those standards. But if you're just a regular user of e-mail, you don't know, or care. You send e-mail, and Uncle Fred gets it. That's all you care about.
You can't, however, handle scheduling a meeting, or a birthday dinner appointment with Uncle Fred that way. Your calendar program might, if you're lucky, be able to send an invitation to ol' Fred; but his calendar program might not understand it. You might want to schedule a meeting between you, Spacely Sprockets, and Cogswell Cogs, but you can't do it, and you lose some valuable time playing telephone tag to set up the meeting.
Wouldn't you, then, pay a little bit of money to get a program which could "talk" to other calendar programs? Or wouldn't you stay loyal to a program which allowed you to do those things? I know I would, and I think you would too. That's why I believe that calendar interoperation will increase sales, rather than lose customers. And if the companies making this software don't believe that, then they're going to lose money as their customers move to buy software from companies that do believe it.
No company ever went broke selling its customers what they want to buy. I believe that the time has come where we want to buy calendar ease-of-use. I hope the Microsofts, Palms, and IBMs of this world believe it too.