I think the crowd helps towards the end. My first marathon was the Edinburgh marathon last year. I was under-prepared and it was HOT.
The main sponsor of the Edinburgh marathon is McMillan so there were loads of pockets of McMillan sponsors who went mental every time a McMillan runner went past shouting their names out etc (as they had names on their top). I ran beside a McMillan guy for a while and it was doing my nut in, so I can only imagine it was doing his nut in even more. What I mean here is - we're both struggling at about the same pace (I finished in 4:48 - it was a struggle!) and every mile or so (or even less sometimes) someone's going "Go on Mark, keep going, you're doing great" and they're banging those blow up stick things together. I didn't enjoy that part of the support at all. It just got really monotonous and I think "Mark" thought the same thing.
What was great at Edinburgh though was the crowds of local people in Musselburgh etc handing stuff out to the runners and qutie a few out with their hoses, giving you a nice cool shower on the way through if you wanted (that was awesome in the 25 degree heat!).
The wall of noise for the last couple of miles are great to spur you on to the end as well.
My second marathon was the Loch Ness marathon, and I much preferred that experience.
I don't think you need the entire course lined with supporters to spur you on - the first half of a marathon is relatively easy if you're even semi-prepared and in fact up to about 16/17 miles should be comfortable. It's after you hit the wall (if you hit the wall) that the encouragement really helps. So, in an ideal world for me, I'd have seen only the hose-toting supporters from about mile 10-18 but nobody else, a crowd of supporters at each mile marker from then on to cheer you past each little milestone and then a wall of noise from about 24 miles until the finish.
Not asking for much, am I?