As mentioned above, some sent email generates a waiting for item (delegations or requests) and gets dragged into that folder. This is the only "processing" I do with sent items and I do that as soon as the item is sent.
The rest of it, at first glance, would appear to be irrelevant to you once sent and could be deleted. BUT, there are a few gems in there that you may need a year from now when some project blows up and people start ponting fingers and looking for a scapegoat. You will retrieve those gems to prove that you did in fact respond appropriately to the issue way back when and if the ball was dropped it was dropped downstream of your area of responsibility (CYA). Certainly the person upstream of you saved themselves by producing the email that passed the buck to you. Now you need to find the email showing that you responded appropriately. That will be in your "sent items" folder.
For that reason, sent mail is a valuable archive to be kept for some retention period (5 years or whatever). If storage space is an issue, then archive it off of your mail server onto a hard drive or burn it to CD or whatever. Digital storage is cheap cheap cheap and takes little to no physical space. Use it to your advantage. Also, there is no point in sorting sent items into "potentially valuable" and "obviously useless." That will just consume your time. It is easier to just archive it all and let the search feature find what you need later. Plus the one thing you end up needing may have fallen into the "obviously useless" category and been deleted.
Think of your sent mail as your most important CYA resource. In fact, it is a good habit to drop extra nuggets in there, too. When you respond to a request by phone or fax, you can send a short email confirming it, like "I faxed the information you requested, please let me know if you have any trouble receiving it." Then, if that issue eventually blows up, you have that little piece of documentation burrowed away in "sent items."
