Here's how it ought to work in my mind:
This subject has come up a couple times on IRC, here is what I've said:
I'd generally like to avoid throwing more decisions in users faces especially ones with long explanations.
If the main decision that the user had to make was a) how much disk space am I willing to contribute to bitcoin [knowing it can be reclaimed anytime if needed], and b) how much bandwidth do I want to contribute, then we could probably get users to make the right decision every time without burdening them with the details.