Perhaps the fundamental error people make when thinking about backups is thinking about backups.
Backups are not important in themselves - it's the restores that matter. (So backups become important simply because without them you can't do the restores.)
We can divide the files on your machine into two piles. The dynamic files - the data - that change all the time, and the static files - the OS and applications - that only change when you apply a vendor patch or update.
With the static files, then yes, you do not in theory need to back them up regularly because they're not changing; you have the original installers and can download the patches and re-do any configuration and get them all back. So, you can do a restore, and as I said, that is what matters.
However, there are various reasons you should be backing the static files up anyway:
- How long will it take you to put everything back? It will usually be quicker to do a restore of a full system backup than reinstall and patch everything, and then do a data restore. You're doing this restore because the machine is dead, remember.
- You can usually pull a single file out of a full system backup if that's all you need - again much faster than a rebuild.
- Doing different backups for your static files and your dynamic files requires you to make sure you don't put a file in the wrong pile, which adds complexity and the possibility of error.
With the dynamic files, your strategy isn't very good either, because when it comes to a restore, you might not have a backup (only most of the files are in sent items) and even if you do, how old is it?
There are also a couple of considerations that apply to both kinds of files.
A good backup scheme is as automatic as you can make it, because if it is possible for you to forget, you will forget, and then you won't be able to do a restore. At worst, your only interaction with your backup system should be rotating the media occasionally and, of course, testing restores occasionally.
Actually let me re-emphasise that - you need to test your restores. It doesn't matter how good your backup process is on paper, unless you're testing restores you have no way of knowing if it is working or not.
Also, backing everything in one go up is simpler, and simpler is always better with a backup system, because there's less to go wrong.
Lastly, there's an old rule of backups called "3-2-1". Something is not properly backed up unless you have three copies of it, on two different kinds of storage, one of which is off-site. So even having a full system backup process in place is not enough - you need to at the very least be rotating some of the media off-site in case your office burns down.