alt Hacker NewsI'm not sure that's paranoia (as others have pointed out, OEM firmwares have had security problems before), but sure, let's ignore security problems for a moment.
1. Firmware contains bugs. Old proprietary firmware tends to not get fixes. If you switch to an open source version, you can get the bugs fixed.
(Edit) 1.a. Old proprietary firmware also doesn't tend to get new features, and open source replacements can cover that. (eg. booting over HTTP(S) or security features to help against Evil Maid attacks)
2. Libreboot claims to be faster to boot than the vendor firmware. Depending on the particular device/firmware, that wouldn't surprise me at all.
Yes, I said in another comment that I might have used the wrong word. It's still not something I have a lot of motivation to do something about. At least not until the process is easy.