So much depends on the individual. I was never very fast, peaked at 2:45 for the marathon.
When I first started running 80+ mile weeks any run over 15 miles was very difficult, and tended to have a lasting effect over the next several days. I made it a point to sacrifice these workouts in favor of a long run once a week because I thought that element of my training was lacking.
By the books, I was better at racing over 5k and 10k than the half and full marathon, yet I was concentrating on the longer distances.
After about 1.5 to 2 years of training for these races the long runs became much easier, and endurance was no longer my shortfall. I thus rarely ran more than 18 miles in a single run, and more usually it was about 16 miles.
I was also running more miles overall, generally in the range of 90-100/week.
In short, for an average Joe, I think it is very important physically and mentally. Especially for those folks who are making their first attempts at racing longer distances.
Over time, the more used one gets to running mileage day in and day out it no longer becomes vital. When you are averaging 15 miles a day anyway you are getting in plenty of aerobic work. Also, those runs in excess of 18 always left lingering effects for several days. In short, the squeeze no longer became worth the juice.
Granted, I never played around with long runs of greater than 22-23 like some do. Perhaps I would have benefitted from those very slow 25+ mile runs that some advocate.
Didn't Frank Shorter do 30 mile runs at (for him) a very slow pace?