I sort of agree with your statement. Mayonnaise isn't really that healthy considering how many calories are in a serving of it, but as long as it doesn't throw off someone's calorie count, then who are you to judge? This is really just my opinion, but I think it's important to not have a boring diet full of boring food. Why? Because you just end up tired of eating stuff that's not that exciting or doesn't have the flavors that you're used to. In a way, dieting is as much mental as it is physical, which is why I talked above about people using weight-loss surgeries kind of eschewing the whole mental process.
Don't get me wrong, I try and avoid stuff like mayonnaise or certain salad dressings. If I have to use them, I'll go for light or fat free, which can have a significant difference calorie-wise (25% of the regular variant or less). If I want to add a little spice to a sandwich, which I rarely eat, I would go with something like hot sauce, which usually has no calories.