Well I actually prefer to make my pioneers pretty tough so they can go off into fogged areas and hunt good spots to settle, as my main forces are normally clearing monster lairs, or doing quests and can't be bothered escorting missions for the basically helpless basic pioneer.
I think most of the time you would be better off training one spearman and one weak pioneer seperately. Still, I'll grant that there are times when you want Hardy Pioneer, and that's cool. There are still plenty of times when you have some routine task suitable for Weak Pioneer, like in the midgame you already know the site is clear.
It's a little bit hard to express why i don't like this design but I'll try. The things that bug me is that (1) the AI will probably never use Weak Pioneer even though sometimes it's optimal, (2) because all traits are open it kind of kills immersion. One of the races has a "special" unit Conscript Pioneer that thematically seems like it is to be a special perk, but on turn one of every game you could choose to make an even more-specialized and therefore more optimal unit. It's really the whole unit design concept, although Pioneers is such a simple case that the default unit is provably non-optimal. Analogy, It's like if in a Medieval game it said: the English and Welsh have these great longbowmen, but if you want you can push the "Have Even Better than Welsh Longbow for the Same Price" button on turn one. If you don't like it then don't push the button. Huh?
