While the elite / professionals are not extremely bothered / impacted by the hoards of walkers and joggers in most marathoners, all sub elites are impacted in the following ways:
1. Getting a bib number - Most big marathons fill up very quickly. Runners who run between 2:20 and 3:30 have to either register super early or win a lottery to participate. This makes creating contingency plans difficult, and ensures that not all the most qualified runners participate (it is a race, shouldn't a runner expected to finish in the top 500 of a race have some sort of priority over a walker in entering?)
2. Date / logistics - You see more and more marathons being run in warmer temperatures, partially because the ideal weather for the walkers is a lot warmer than the ideal weather for competitors
3. Costs - One of the big costs involved in a race is closing the roads / police protection. The fact that many big city marathons last 8 hours instead of 4 doubles this cost, which is borne to the competitive, non-elite runner in higher entry fees
4. Destroys competitiveness of a race - With so many walkers / non-competitive runners in a race, the idea that a marathon is a competition is lost on a lot of people, the press included
5. Walkers steal publicity away from the better runners. Here in NYC, old timers frequently mention how Central Park race results used to be posted in the NY Times, and how much of a motivating factor it was for many competitive runners. Now, these stories are replaced by feel good stories of people running a first marathon. Do not underestimate how big an impact this has had on the competitive level in races.
To counter previous posts and fallacies:
1. 'Increase in entrants increases the prize pool' - most prize pools come from sponsors and are unrelated to the number of entrants
2. 'Walkers / joggers in race contribute a portion of their entry fee to charity' - Again, corporate sponsors make by far the biggest donations. I was shocked when I ran the Philly Distance Classic a few years ago and saw that less than 10% of my entry fee went to charity.
3. 'Of course people know that running a 2:30 marathon is better than running a 5 hour one' - Again, I think this statement is false. While most non-runners know the 2:30 guy was faster, that is 100% negated by the fact that the 5 hour guy was out running for five hours, while the 2:30 guy was running only half as long. While people can possibly see running for 150 minutes, running for 5 hours is just insane! Hence, most people see the accomplishment as the same, as both completing the marathon.
Note: I am not bashing slow runners. I have no problem with what walkers or joggers do or what their priorities are. If someone wants to spend 10 hours completing a marathon and raise a few dollars for a charity in the process, go for it. What I don't like are race directors that try to cater to everyone, as the sub-elite runners usually get the short end of the stick. I still have no idea why the NYC Marathon isn't a 100% walkfest. There is no need to invite elite runners or have prize money, so why not just end the façade that this is a race. The level of interest / number of competitors / amount of tourism dollars / amount of charity contributions would be the same either way, and without a prize pool they can donate the prize awards / under the table appearance fee money to charity instead.
For sub-elite runners, the current marathon system is incredibly broken, with race directors and politicians (not slow runners) to blame.