Every pitcher is capable of giving you 170 innings unless they are injury prone and miss a ton of starts. That's not a skill. That's just showing up to work. Staying healthy is a skill, but once you're healthy, going 170 innings isn't impressive. 170 innings is a very low innings total for someone who lasted all year in the rotation. The reason starters don't get to 170 innings, is because they pitched badly, and weren't given more chances to start.
80 pitchers made 28 or more starts last season, more or less lasting the entire season in the rotation. Only Phil Hughes, Mike Pelfrey, Scott Kazmir, Jeff Locke, Jorge De la Rosa, and Dan Haren pitched fewer than 170 innings.
Norris is pretty good at getting batters out. He's a solid pitcher. He's not good at all at pitching deep into games. He has been good so far at avoiding injury.
Every pitcher isn't capable of giving you 170 innings. As you mentioned, the first part is showing up every 5th day... and plenty of pitchers are incapable of that. The second part is performing well enough, to continue to get the ball. Norris fits both of those criteria, that's why he is better than the typical dregs most teams teams have as their 5th.
You act like 80 pitchers illustrates your point... while I don't think it does. When a season starts, there are 150 starters across the league. That 80 out of that initial 150 got to 28 starts, illustrates the value of having someone consistently go to the post. Beyond that, you have a team like the 2013 O's who trotted out something like 13 different guys to start games.
As you mentioned before, most of the time you aren't going to have 3 guys who can stay healthy enough to stay in the rotation for the season. When your 5th starter is someone whose shown that consistency for the 3 previous years... that's a large part of his value.
Him not pitching deep into games means little when you are comparing him to other 5th starters. It means more when you compare him to better pitchers.
