I suppose it depends on how you define valuable.
I tend to think of valuable relievers as ones that strike guys out, make batters swing and miss (ie limit contact), limit walks and when they do allow contact, it should be weak, a ground ball and not a HR.
So by the numbers I look at and compare everyone on the Jays roster, Little wins that title hands down even though his walks are starting to be in the real problem area. They generally dont hurt him though because he'll just strike the next guy out or get him to roll over.
Yariel is running a .200 babip against right now... which is probably unsustainable, strikes out far fewer batters, gives up more HRs, though definitely walks fewer as well. Little is nearly 60% ground balls when he's not striking everyone out. Yariel around 41% ground balls.