Have you noticed getting access to unique items you never saw?
Not unique in the sense that they cannot be obtained without mercantile, but the special stock often includes craft-only items such as MK IV mines/grenades, MK II EMPs, chemical pistol ammo, rare drugs such as focus stims, etc.
Having high mercantile can actually be a quite good investment if you want all of those- it's far cheaper than increasing all the crafting skills. It's kind of a jack-of-all-trades deal, you get access to a lot of craft-only stuff with less skill point investment overall, but on the other hand you don't get all the benefits crafting would give you.
I wish there were unique items that would be only accessible with high mercantile, preferably as quest rewards. Or maybe even unique outcomes that are something more than just bonus money. It would give a lot more incentive to spend skill points on it.
higher quality items that would would have a much much more difficult time finding/only located in late game.
I definitely saw more components of higher quality, but it wasn't anything special. I'm not sure if the special stock components have higher quality or if it's just that they are more likely to appear and as such you are more likely to get something of higher quality. Overall I think alone it's not enough to justify taking mercantile.
I also think one of the problems is that traders that offer special stock are pretty rare- maybe a little more than half a dozen in the entire game. This somewhat diminishes the benefits of having high mercantile, unless you don't mind waiting for them to restock.
