Most modern web-based email systems have decent spam filtering (I've used gmail and yahoo). Anything that looks like spam gets filed into a "spam" folder. Pretty straightforward, right? One thing that strikes me about these systems is that the spam folder has a "Delete Forever" button, while normal mail folders have a regular "Delete" button. The difference is that the "Delete" button sends deleted messages into the trash, while the "Delete Forever" button obliterates the spam completely. The "Delete Forever" button should be replaced with a regular "Delete" button for the spam folder, and here's why:
The purpose of the "Trash" folder is to give you a safety net in case you delete something you didn't want to. When you delete something, you have the option to pull it back out, provided you didn't empty your trash. This gives you a "2-level" delete process.
The "Delete Forever" button, however, does not provide this same level of safety. Once you press it, everything is gone, forever. There's no option to go back in and grab something you accidentally deleted. It's a "1-level" delete process, with no safety net.
These differences are very important when you consider the usage pattern of each type of folder. When you read email in a "normal" folder, you generally do so one at a time. Since spam filtering is pretty good, you generally don't have to deal with large amounts of spam. In this usage scenario, you go through each message one by one, deleting the ones you don't want. Generally, each message is vetted, and you can be pretty confident that what you delete is what you expected to delete, AND you still have that safety net.
When you "read" your spam folder however, the usage scenario is very different. Instead of reading one by one, you generally skim quickly through the subject lines and look for anything that jumps out as "not spam". Since it usually is all spam, the easy thing to do is hit the "Delete Forever" button. Unless you delete spam on an hourly basis, you can easily have hundreds of spam messages in just one day. Skimming through all that is virtually impossible, so the quickest and easiest thing is to hit "Delete Forever".
This causes uncertainty about what is actually being deleted. The uncertainty is created by the fact that spam is detected automatically, and because it's an automated system, it can be wrong. We've all run into the situation where you're missing an email, and it shows up in the spam folder. In that case, if you had "Deleted Forever" first, you'd never find that message.
So now we have a situation where:
That's in contrast to the situation for "normal" folders where messages are checked by you, and have a safety net when deleted. Doesn't it seem like this situation should be the opposite? The automated nature of the spam folder, as well as the bulk of messages usually inside of it, is exactly the type of situation where you need the safety net of a "trash" folder. You could even say that you need it more than for non-spam messages, as you normally are not looking through every spam message one by one.
The fix is simple. Remove the "Delete Forever" button, and replace it with a regular "Delete" button. Spam messages should go into the trash, just like any other deleted message. Then you can empty the trash whenever you like; when you're sure you're not missing anything.