You should change whatever of your own stats you find fun or enjoyable, but having one creature in the temple for a few seconds keep 3 save from scavenging for several minutes will make scavenging against another keeper completely useless.
A scavenger room is expensive, it needs high level creatures to be effective, they need to be in it all the time and it costs money to use it.
With your change, if your rival decides to scavenge, you build a temple and can continue to turtle. The intend is that you can only keep your creatures save in the temple so you can't go stronger and you'll have to attack or out scavenge him, in which case your rival has to dig out and attack you.
For vampires this isn't a bug with intended behavior, combined with the high scavenging skill of the vampire and that his primary job is to scavenge it is basically determined that only one player gets vampires.
In recent versions of KeeperFX you can use the 'must obey' spell to force your vampire to obey your command to go pray in the temple, but this makes him really upset.