Walker brings some things to the table and takes some things off the table. He's no miracle cure.
Nurk is almost washed and can't jump, but he still has a lot more ball skills and is a much more effective high screener and handoff guy (being a wide body with a totally unmovable base). I'd argue his passing in tight spaces is one of the big reasons why the offense is humming this season. Any team with Lauri on it needs a passing big. Nurk can also make a quick floater / push shot off a short roll, which is a very valuable skill in today's NBA, with crazy long rim protectors ready to pounce.
Kessler is obviously a much better paint defender and works harder overall. He's also a better rebounder, but the difference is smaller than you'd think, considering Nurk is ground bound. On pick and rolls, he's totally useless unless it's an alley-oop.
IMO one of the most valuable things Nurk brings to this team, even in his current form, is that he seems like a guy who knows all the dirty veteran tricks and is something of a hard-***. I wouldn't call him an enforcer or anything, but at least he's not soft and moody like Kessler, who you can push around. On a team of quiet nice guys, that can be important.