I think tomato sauce (ketchup) provides a much better contrast. The acidity counteracts the oiliness and the sugar in the tomatoes and salt on the fries rounds out the flavor profile. You get nearly all the flavours in one meal. Even better with curry ketchup. Mayo, OTOH, doesn't provide a strong enough contrast. It's sweet and tangy, but still predominantly tastes like fat to me.
But enough people like it, so there has to be something to it…
* lots of people eat fries with 'fry sauce' which is mayo and ketchup mixed 1:1. i'm a fan of this. why choose?! this is popular in many western us states and i'm pretty s
* mass-produced mayo is not nearly as good as fresh mayo
* sometimes people really mean aioli when they say mayo, and oftentimes aioli has that citrusy tang that counterbalances the fried fat
* sometimes i dip my fries in mustard, which also has the tang
* same with vinegar (usually with fish & chips)
Aioli is different. It is also an emulsion, like mayonnaise, but it's mostly about the garlic, and in some cases doesn't even contain egg yolk (but it usually does).
The inevitable mixture of monkey-gland sauce with mayo when eating steak, is well, nothing short of glorious.
It's just a name for a mixture of a whole bunch of ingredients, notably onions and chutney.
Here's a recipe: http://www.food.com/recipe/monkey-gland-sauce-266015
The abundance of it in commercial establishments, however, is a definitely a Utah thing. Unfortunately, the Utah product is rarely homemade and usually a mix of Best Foods and a Heinz product, and isn't anything nearly as creamy as you'd get in the Netherlands. That said, nothing beats Heinz Ketchup. If more places in Utah took just a bit more pride in the fry sauce it could be a much better product on its own.