Some quick thoughts I'm having:
1) Using iPhoto across accounts means that you must quit iPhoto every time you're done with it. Translation: You'll constantly be switching to the other person's account to quit iPhoto so you can use it. A small issue, but crap like this adds up and makes using the computer a huge annoyance if you have to spend 5 minutes "fixing" everything each time you try to sit down and work.
2) Having to manually add music to the other iTunes account is a big deal because there will always be times we forget to do it. Thus our libraries will become more and more fragmented over time until we never know what's where.
3) If you're logged into one account, you won't hear a sound when the other one gets e-mail, right? How do you decide who gets to stay logged in and have that benefit while you're not using the computer?
4) You can't open Final Cut Suite software on 2 accounts at once. This creates a similar problem to iPhoto...I have to remember to shut down Final Cut when I walk away on the chance that someone else will want to use it.
That's only 4 things, but with computers I find that I always run into twice as many problems as I was expecting. It just seems like it would grow into much, much more trouble once I actually tried anything.
You may not find these issues problematic, but that does not make them non-existant.