Sadly there are many ways to cheat (like using macros), that are hard / impossible to detect. That alone would not be an excluding factor to a rule for me. 

Since metronomes are allowed in many other games, it's reasonable to ask, why they are banned in Gothic. If they would be allowed and some people would start using them, it would put others in a disadvantage, if they would want to have the clean Gothic speedrunning experience without help of external software. That's my personal reason, why I slightly lean towards keeping them banned.