Its definitely an area where you get the most out of gaining the attributes where it enhances an existing forte. But in combination with the right player.
So a Black Orc gaining ST is far more use than a AG gain. A Human Thrower gets more out of AG than a lineman.
But there are times when it can compensate something lacking in a team. Largely though I think this is most useful where its something the team is at least "middle of the road" with, rather than deficient.
An elf gaining ST adds more, in my view, than a Goblin, for example.
A Human lineman gaining AG adds more than an Elf lineman doing the same.
Its not that the others aren't good, but the benefit is not quite as large.
Of course, sometimes its a combination of both.
Ask me to say who gets more value out of AG between a Human Catcher and an Elf Catcher and its tougher - they both can do a lot with it.
Similarly ST is still really handy on a Human Catcher, even though it only brings him up to a moderate level. Largely because he is a weak link, rather than in, say, a Halfling team where it would be one in a whole team.
Overall though, I'd suggest that the more hybrid teams gain the most out of stat increases in total, whilst the other teams can gain with applying it more to specialists.
So for example I believe Humans or Necromantic, for instance, would do more with a few AG than Elves and a few ST more than Orcs.
Similarly, in case the Goblin receiving the +ST boost is a regular non-mutating goblin, dodging into cages on a 3+ with a re-roll and getting a 1D-block against ball carriers or other players not protected by Guard pieces is still potent. 
