Lack of a health bar is only useful when the amount of hits to the boss is small and predictable (like how the Sonic games always had 8 hits for the bosses, final ones aside), otherwise show a health bar or something. Players like to be able to know how much progress they're making.
I agree with this view. If you have long, drawn-out boss fights, I definitely want to see a health bar of some kind. If your boss fights only take a little while (scaled accordingly for your game's balance), I don't necessarily need to see it - but usually I prefer seeing it.
I don't have a strong argument for why I prefer seeing it. I just, as a player, want to know roughly how I'm doing against a boss. Part of that measurement is how much damage I've dealt in comparison to how much it has.