The Bears signed a guy with versatility that adds speed to the offense and creates mismatches when on the field with Cohen, Gabriel, etc. Add to that the fact he was 3rd in the NFL in kickoff returns, 50% better than Bears kick returners. He won't be the primary weapon on offense by any means, but the fact he was used as WR, RB and his kick returns make him more than "filler" by a long shot.
Why do we consider a RB as "filler"? Because he is not a big name? Fans would rather have a name than a fit. (I don't mean Leveon Bell - he was superproductive, but too expensive for us). but fans would have been fine with Ingram who is Howard with fumblitis. Davis in the backfield will not be just filler.
Skrine? Well, I do expect he more or less is filler in the sense that he will play where Callahan played. I don't have reason to believe he is a great signing by any means, but I will just say that he sucked on the outside but won't play there. Callahan was not as good there either. (That's why nicklebacks are nicklebacks) But you do need to put someone in that spot, so if they were able to do it with a FA, it saves a draft pick in a year when we don't have many. But, no, Skrine id not some significant addition.