What benefit is there in turning your spouse into a Vampire?
The only benefit from the hearthfire DLC is making a coffin more accessible by placing one in the basement of your house (though this doesn't depend on having a vampire for a spouse, just if you are a vampire yourself).
Turning a spouse into a vampire does not have any explicit benefits other than what you stated. Most of the time the only difference is that their eyes change color and that's glitchy sometimes.