Who uses it?
It seems as though there’s no reason not to pick it, but I rarely bother anymore, because although those with the perk tend to reach level 11 faster than others who started at the same time, they don’t tend to reach level 11 before the others have reached level 10, and so (for most of the way) they have the same number of “active” perks that actually affect their effectiveness in battle (and thus their ability to gain experience). So usually, at some critical point when the core of the company is around levels 5-6, you’re thinking “most of us really need both perk A and perk B to take us to the next level … if only I’d chosen an active perk at the start; then I wouldn’t have the headache of choosing which one to take next”.
Wildmen start with -15% experience gain, but it’s not uncommon for talented wildmen (who are better able to wear and utilize quality equipment) to progress through the levels just as quickly as your starting companions.