Barter 70 for pack rat? I wouldn't really call that the worst deal.
Can be damned useful for someone who would prefer to make the game harder by going with fewer, or simply without followers.
And later on, the big-gun ammo.. .even just the .50 cal ammo gets pretty damned heavy. Unfortunately it doesn't even help with missiles, at 3lbs/per
Given, my char with a Str of 7 is hardly a wimp, but Pack Rat (and strong back if you have the END) go a long way to mitigating a low strength

One thing I tend to do with the game is, for looting purposes.. is compare the weight/value balance
If it's 1 lb of weight.. I generally try to make sure value is no less than 20 caps. And overall I work to keep that balance of cost. Most bang for the buck you could say.

I know I saw more than four. I saw enough that I decided to start raising Medicine earlier than I normally would have to try and take advantage of them. The only ones I can remember offhand are dealing with the pet mole rat in Sloan and helping the soldiers in Forlorn Hope.

It also seemed like the necessary skill level that any of these needed kept going up as I leveled up, but I don't know if that was just chance or if the game's designed that way.