I believe the problem with that is some sort of evil maid attack. Let’s say you leave your laptop unprotected in your room, someone goes in and changes the boot partition into an altered version that will record the disk encryption password. The attacker can then retrieve your password and read your data.
If you are using PureBoot, you should be able to detect that this attack has happened (thus preventing you from typing your password into a compromised system). However, you will not be able to prevent the attack.
With /boot in a separate USB, you can prevent it by keeping your boot partition close to you at all times. Of course, this just moves the problem and you must now protect the boot drive from any threat.
Also, I don’t know if Heads (and by extension PureBoot) is able to verify the integrity of the boot partition in a USB drive. If not, you can not verify the integrity of /boot and this is a bad idea.
It should be noted that a generic attack on /boot does not require physical presence. However, if you are able to compromise a system to such a level that you have write access to /boot, you can probably already read all the information from the drives, so stealing the disk encryption key is probably useless.