This might go without saying, but people don't usually sit in bathtubs unless they're taking a bath. It'd be odd if you visited your friend's house and they offered you a seat in their bathtub. "Come on in, make yourself at home! It looks like there's an open seat in the bathtub if you'd like to sit down ..."
Believe it or not, Custom Tub and Tile Resurfacing doesn't think this idea is all that crazy. When they found a nasty old clawfoot tub, they saw its hidden potential and decided against throwing it away. Instead, they converted it into a stylish sofa that guests would be happy to use. Their project takes a bit more elbow grease than your average hack, but if you have the time, tools and old tub to pull it off, go for it.