It's only 1 year... For some schools it's a low risk, high reward deal. It's not like you have to commit to a 4 year scholarship, it's only 1 year. Worst case scenario, you give him a scholly, he gets hurt, never plays, and he's gone in the spring. Best case scenario, he's healthy, has a Heisman like season, and leads your team to the CFP.
The key piece for all teams would be who they have currently at QB. If you have a young guy with a bright future, (UCLA, BYU, etc.) then you are right, it doesn't make as much sense.
But if as a head coach you have no idea who your quarterback is going to be in 2016, and don't have anyone immediately in the pipeline, then a 1-year risk on Taysom is like mana from heaven. It buys you one more year to recruit the quarterback of the future, while having a guy who can immediately make any team in the country better, (outside of maybe OSU).