I too wanted to justify the expense of a .Mac account. So, I decided to do a little exercise to determine it's worth (at least to me). Here's what I found...
What .Mac gives me for $99.95/year or $8.33/month:
-100MB of server space including FTP and WebDAV access
-Access to an IMAP email account (or POP if you prefer)
-A ".mac" email account that's not tied directly to my ISP
hmmm... doesn't sound like much, but wait there's more.
Let's look at what .Mac combined with OS X allows me to do with relatively minimal effort.
The ability to EASILY...
-Access my Address Book online
-Create web photo albums with iPhoto or other 3rd party software
-Create file sharing web pages with Apple Homepage
-Create iCards from any photo stored on my iDisk
-Backup important files to a non-local storage area maintained by Apple
-Synchronize files between work and home, including Address Book, iCal, bookmarks, PDA and Cell Phone
-Utilize McAfee anti-virus software that is updated automatically
-Post and share Calendars through iCal
-Share personal photo slideshows (as a screensaver) with other Mac OS X users through .Mac Slides. If you have kids this is amazing.
-Share files across the internet with other Mac OS X and Windows users through a public folder on your iDisk.
-Post your own Blog with 3rd party software such as iBlog
Wow, not too shabby.
Now what would it take, in terms of time and effort to be able to do all of this without a .Mac account. It's all definitely possible to do for free, but personally I'd much rather be spending time with my family than trying to develop all of these features on my own.
Of course, this does not apply to the hard-core developer who revels in his own ability to create the same functionality from scratch. Again, these are mainly features of convenience that are designed to increase the usefullness of your Mac with little impact on your time and effort.
$99 a year? I think it's worth it.