Johnson historically has only run about one NW race a year - generally I think it's a promotional contract with his sponsorship. Kenseth, Logo, Hamlin, Harvick, Keso, and a few other key cup guys have contracts where they 'split' rides for an owner so they only get in maybe every few races. Kyle Busch pretty much runs them all. Junior ran 2 I think this year, and as I recall only one last year - usually on a plate track.
Whether they gain any advantage for the cup races the following day is always a matter of opinion for each driver. Some say they do, others say it's not an overall benefit considering they have to dedicate time and focus over the weekend which detracts somewhat from their cup focus. The tire compound is really the key factor as to whether they think it's a benefit or not. On some tracks (but not all) and for some tire compounds (but not all) the knowledge can be quite relevant. Not only the issue of 'focus and concentration' lost over the weekend but the obvious factor of additional risk is considered by drivers. And I'm sure some have contract restrictions that disallow it because of the additional risk.
This weekend at Phoenix NW and Cup are on the same tires. The left sides are different than the left sides they ran here in the spring race (right sides are the same as they ran in the spring). So for those cup drivers running in NW they may gain some important data on left side tire behavior. And I'm sure that data will be made known to the other cup teams of the same owner.