What is useless to you, can be usefull to someone else. That is called Roll Playing. Just like most people don't pick Mercentile in TES games, it's still there for the few who do want it or want to RP with it.
You don't need a perk/skill/game mechanic to role play. If a perk is useless from a technical point of view because it doesn't give you any useful advantage while playing the game (compared to other perks or generally) then it's useless. Period. Mercantile was useless because money was abundant in Oblivion. You'd have more money than you could spend after a few levels, whether you had a mercantile skill of 5 or 100. So from a gameplay perspective the skill was useless (unless you used mods like Enhanced Economy, in which case mercantile became useful again). If you wanted to play a merchant type character then you could have done so without the mercantile skill. Just like you'll now have to play an intelligent character without an intelligence attribute.
i used here and now a lot (well occasionaly at least) when im close to haveing the level i need in something but dont want to waste time hunting mole rats in the waste land (and most of the perks were not very useful to me), the 10% exp boost is one of the most useful in the entire game, helping to level fast is alot more useful then being able to silently kill someone while they sleep, a pro would just use that fancy new rifle from the top of the biggest tower in the area anyway :gun:
If your goal is to play through the game as quick as possible then yes, the perk is useful. If you play through the game slowly and if you know you'll reach the maximum level sooner or later anyway then picking this perk is equal to wasting a perk.