A well made unglued dovetail has considerable mechanical strength, but there will inevitably be movement and racking (no matter how slight) that will gradually weaken the joint, though perhaps not to failure - look at old quality furniture that used hide glue. The glue has failed, but the dovetails are still sound, if a bit loose. Nevertheless, I's still prefer to use a modern glue - can't see how it would weaken the joint.