The problem is Outlook and Exchange. The users see the mail client, calendering, and the like, as essential. The word processor and spreadsheet are secondary to that. Once some exec starts talking to sales about getting just Outlook, they are sold on the wonders of getting the whole MSOffice suite.
If you look at Microsoft's pricing, it's fairly obvious why. If you're first getting Outlook for 135 euro then another 135 euro to get everything else is an easy sell-up, particularly since I'm guessing the sales reps will give you a volume rebate on the Office suite but never on Outlook alone. For at least a decade I've heard product after product being called "Outlook killer" but they all seem to fizzle and my impression mostly because they focus on being POP/IMAP clients. Calendaring is probably more essential to an organization, and I don't mean the simple one-off meeting.
When are people available and what meeting rooms are available. Setting up recurring meetings (like say a weekly staff meeting) that lets you easily modify single instances (because this week is easter), calendar sharing, forwarding events with proper notification to the meeting owner, overviews of who will/will not attend or haven't answered, including the agenda or attachments, corporate directories, personal directories, all that practical stuff like that if I start writing a mail to someone in-house it warns me right away they're going to send an away message instead of waiting for me to send it, get the auto-reply, realize what I just send won't work, then another email to say forget that, let's do something else when you're back on Monday.
Geeks hate meetings and scheduling, every one of them myself included. Good calendar software which makes it easy to drown people in meetings is just begging to be swamped with them so it's not exactly an itch we'd like to scratch. We're very busy trying to invent and push non-meeting solutions like email or IM and claim we're solving it better. I'm not going to fire up debate, but the fact of the matter is that getting all of the people involved in the same room at the same time to discuss/decide matters is still a very popular idea. And if you want to get rid of Office, you need to get rid of Outlook and if you want to get rid of Outlook you must handle this well. I'm sure there's lots of people who'd like to drop Exchange and the CALs, using non-MS products despite still sending around MS documents so it should be easier than taking down all of MS Office at once.