I've clocked over 180km, and most of that has been going on walks and catching coincidental Pokemon. Yeah, it's fun. However, I can without a doubt say that it was much more fun when you were able to track down rare pokemon. I live in a small town and the 'coincidental' pokemon are 90% Zubat/Pidgey/Spearow/Ratata/Weedle/Caterpie. It's only fun for so long to catch them. When a Squirtle or Electabuzz or Abra or Machop or really anything else shows up on the nearby list, you want to find it. Tracking them down IS the game imo, finding them is where the joy of it is.
And the nearby list makes it impossible. Me and my gf can be walking right next to eachother in the same direction, and that rare pokemon will get closer on her list and further away on mine. Most of the time we even have completely different set of pokemon on our lists. And living in small town where not a ton of rare pokemon spawn and there's not a ton of pokestops and they're not constantly lured.. it just feels like a huge longshot that I'll ever stumble upon enough Bulbasaur's to evolve one for example since I've never seen even ONE in the wild.