Depth has value, but cap space also has value. Nelson Agholor has the fifth-highest cap hit of any WR in the NFL this year. You can maybe live with that if he's one of your top three WRs, but if he's behind (as seems likely) Parker, Meyers, and Bourne, in an offense that figures to play a fair amount of 2 WR sets, that seems like way too much to pay for that kind of depth. They shave off $9-$10 MM if they can trade him (only $4-5 in a straight cut), assuming they can find someone willing to take on that deal.
The other reality is you only have 53 roster spots. Thornton isn't getting cut, so if he's WR5 it's basically a redshirt year. But they've also got Zappe and Strong, rookie 4th rounders, who figure to be healthy scratches most of the time. Can you carry three offensive rookies as healthy inactives for 17 weeks? Promoting Thornton to WR4, where he'd fill Harry's 20-25% of the snaps, seems like an easy solution, providing you can trade Agholor.