is means signing is not necessarily a bad thing! If you just want to make sure you get official updates, signing is for you.
So... what do 'we' want?
We, being the community of Android users who love to modify their phone, basically want bootloaders to follow the model that Google employs in its phones.
You can choose, by typing a command in an adb shell, whether you want your phones bootloader to be locked or unlocked. In its locked state, it will check signatures and make sure everything is official. Great for your average customer, who just wants peace of mind. In its unlocked state, it allows any custom modification, like CyanogenMod, to pass through.
When we refer to a locked bootloader, we mean one that is in its locked state, and usually also that the manufacturer didn't give us any option to unlock it. So when people say they've loaded an engineering version of a bootloader, it usually means they've found a way to load a bootloader made in the development of the phone, which didn't check for signatures (unlocked by default).
So we don't want unlocked bootloaders, or non-signing ones; that might be bad for the average customer. [TL;DR:] We want unlockable bootloaders. Note, the unlocking process shouldn't be something a normal person would be able to get to, or automatable. It should be a choice that a technical user can make.
Edit: What I think the ideal bootloader's functionality should be:
- It should not be replaceable, or only replaceable by a signed manufacturer update. The rest of the phone should be.
- It should have a locked state, where any updates to the phone are checked first (through signatures) to see that they're by the manufacturer
- It should have an unlocked state, which allows any update to the software of the phone
- These states should be switchable by a technical method
- The bootloader should be able to tell what software is on the phone. It outputs a string, say, which includes a nonce and a signed answer to this question. The manufacturer can ask the customer to give them this answer from their phone. If the answer matches up with the signature of an official version of their software, then they can give support and/or warranty to the customer, because they know the software is in a certain state. If it does not match, they know custom software is on and they don't have to provide warranty and support.
Well there you go. Everything you want to know about this bootloader business.