They're in the "We can be whatever we want!" denial stage.
Well, you
can be whatever you want. It's just that being a werewolf isn't as useful if you have lower health and one-handed skill, and being a vampire is terrible if you never use stealth.
Both are fairly detrimental to the archetypes they don't fit. Being a werewolf prohibits ever being well-rested from sleep and the Beast Form power, while powerful, prohibits all forms of healing other than eating corpses while active. It also seems you can't cancel it prematurely and must wait for its time to run out. Vampires are weak to fire, plus they lose all regeneration of and take penalties to health, magicka, and stamina in the sun. Those are serious penalties to deal with.
They are
somewhat useful to any build though. Werewolf's Beast Form will still be mediocre for non-warriors and they're immune to disease. Vampires resist cold and get immunity to poison and disease.