I looked at the mail I really wanted to read from the most reputable entities (banks, government, business partners,...). Usually there is a logo at the top and the rest is mostly plain text with minimal formatting. A nice, colorful email, sometimes from the same companies usually mean one thing: ad.

This is no different from snail mail, where important communication is usually on standard white paper while ads are high quality prints on nice goossy paper.

And obviously, "average users" notice the pattern. Just like my father who almost trashed an important tax-related mail just because it looked too nice, he thought it was an ad.

So the "average user" prefer to see nice ads, I can get that. But what the average user really prefers no ads at all.