The guys in my league that were drafted after Hartline, or went undrafted, and have outscored Hartline are:
Keenan Allen (120.2 to 109.5)
Riley Cooper (117.2)
Julian Edelman (110.6)
Jerricho Cotchery (110.1)
So "throwing darts" nets you about 10 points, if you pick the best available "dart". About 7 points if you pick the next best. And essentially nothing after that.
No need to list the dozens of "darts" that fell short of Hartline's numbers... including several that were higher picks.
The problem with this analysis is you started it week 1. You're not starting these guys week 1 unless you botched your draft or have a bad dynasty team. You're not starting these guys until the bye weeks and injuries start to hit or some guys you drafted early didn't pan out.
Cotchery is actually a great example to cite for my case. If I missed on all of my darts and waivers was a waste land midseason I could pick up Big Ben's favorite red zone target and go from there. While I'm burning and churning you're just sitting on Hartline. My end product = the one you've been sitting on all year, but I took high reward gambles earlier in the season for spots on my roster that didn't impact my short term production.
You are kind of assuming here that nobody else in your league will also go after Cotchery, so you can just have him there waiting for you as a safety net. Only way to have that safety net is to have him on your bench waiting for you to actually need him. He is on a bench in my league - has been for a while.
Every league is different, with different levels of competition, different roster sizes, and different numbers of teams. Perhaps in your format this strategy works. In my format, all of your "darts" were on rosters. We are 12 teams, 24 spots, fairly good competition, and probably what makes the biggest difference...Dynasty. Darts are tough to come by in this format. Hartline is a very solid player to hang onto in my format, simply because there are not enough players that are worth the risk to drop him available.
Because of AP's injury this week, I need to move Spiller from flex into my RB spot. Have to move Hartline into my flex spot as a result, and don't feel too badly about it, as he has been performing at a pretty high level lately.
ETA - In redraft, 18 or less roster spots, I tend to agree with you that Hartline is on the fringe of rosterable. Depends upon your strategy...safe, or risky. Also may depend on the rest of your roster. Ironically, in my league like this, he was just picked up for a playoff game off waivers by someone with injury issues...but he was on waivers to your point.