Bramble Vest question.

So, I've been playing a few games on my different account where I pretty much one trick Sion with some friends.

I occasionally build thornmail against heavy AD comps you know, and because of that I buy thornmail. However, like most people, I just rush bramble vest and upgrade it later in the game as a last item.

Is there even a reason to do that? Like, I could just sell bramble vest and buy a dead mans plate or something. Grevious wounds is nice but when adcs are getting like 5000% lifesteal lategame it doesn't do much.

I guess what I'm asking here is that should I treat it like the tank version of hexdrinker or is it actually worth the upgrade?