Brick is a much-hyped word that gets bandied about with-out understanding what it is.
There's two types of brick, hard and soft. Lets go through this to make the distinction clearer.
- Hard this is where the handset will absolutely refuse to boot at the press of the power button. Diagnostics: Dead screen, no power. That is the symptom of a hard brick. Reason: This can happen by inadvertently pulling the cable when flashing a ROM or even an update via the handset's supporting software such as Samsung Kies, Sony's PC Companion Suite etc. That is the worst case, that is why in this scenario, always, make, sure that no cable gets pulled unless told so by the supporting software in question. The other way is this, flashing in that manner when the battery power is low. Hence for that reason, it is recommended to have at least 75% of battery power in place prior to doing the update in that fashion. Its known as hard-brick, and requires specialist cables and stripping the device down to the circuitary board to revive it via JTAG cables.
- Soft - this can happen through a bad flashing of the ROM in which yields the following. Diagnostics: it has power when the power button is pressed, and/or, the screen comes on. Reason: It can be a user error on this part. For example, failing to clear/wipe the data/cache, can cause Android to go into a boot-loop because of the mis-matches with the Android's virtual machine and the apps installed, or more than likely, a corrupted dalvik cache. The other, is flashing a ROM that is not designated for the handset which can cause the kernel to go into a boot-loop. This is generally easier to fix, but hangs, that is known as a soft-brick.
With those two separate forms of brick, it is generally speaking, a scare-mongering tactic.
The one pure simple reason why carriers do not recommend unlocking the boot-loader, is not just for the warranty.
Its more down to the fact, if the handset ever gets stolen, then unlocking the boot-loader will wipe everything, and re-initialize the handset to factory state, thereby all your contacts, call logs, messages and apps gets wiped cleanly. Also, this is to reduce the amount of support in the carriers, with people calling in or ringing up the technical support, "What happened with my handset - I did something and lost my contacts?"
Just to finish off this answer, you can revert it back, by re-locking the boot-loader at a later stage if you wish, that is in the event the hardware got damaged and hand it back in for repairs.