I'm not saying those guys are perfect options as #3 and #4, although there was a time that Kamara probably could have been the bets Slot WR in the league, I'm just saying that we do have other options. But, I'm not honestly sure how much a #3 WR would be used given how the read progressions work in this scheme.
Still, this team could certainly use another WR. But, they really haven't much tried to use the other WRs that they do have. Even in 11 it appears that the #3 WR doesn't get many targets. I suspect some of that is based on game plan and how the offense works with straight progression on reads, but maybe some of it is lack of talent at WR and Adams is a better and more complete WR than Shaheed.
It just comes down to the present and future cost to me.