the fact of the matter is we have no idea what the effects of the last concussion were. He could have had minimal symptoms. They really could have just been extra cautious with him, similar to Muzzin. We'll never know. Bowen Byram has gone through the same thing and has come back and looked better than ever. Getting concussions is extremely dangerous and life altering but it's not always the end of the road for players. Especially with the right treatment, regiments and awareness of post concussion symptoms. I doubt the Leafs would have put him out there if they didn't believe his condition and mental state was manageable or in a good frame.
Additionally we need all the weapons we can get. Kase makes our team better in a lot of ways and can also be used on the PK if needed. He's going to be a huge boost to our offense.