Well, with the Dex save bonus, it'll be bloody obvious because they dodged out of the way incredibly quickly.True.
On the other hand, a foe might not take much damage from a Fireball due to having a high Dex save bonus, or due to having Fire Resistance. Should the PCs necessarily know why the foe doesn't take much damage? Should player knowledge drill down to those mechanics level of details?
With resistance, an experienced player will see the creature get engulfed in the flames and not look as damaged as it otherwise should.
You seem to be using your argument to say that the DM has no need to narrate what happens in a visceral, observable, and perceivable way. That's your prerogative, but from where I'm sitting, all I see is a DM that reduces the color and vibrancy of the world, and the thrill of a character figuring things out through hard-won experience, in a way that reduces their effectiveness.
Basically, if a being is resistant to fire, and you never give them a hint because *reasons*, the fights will go on longer than they should and they'll suffer more damage, burn more resources (pun intended), etc. and that IS a functional, negative, material penalty because of it.
No creature is resistant to everything. By withholding any hints or information you're hurting players that could switch tactics and win much more quickly.