Unless your a dune rider, or a very talented MX rider... bottom end can mean a lot. Especially for the trail riders. Staying on the pipe all the time can give you quite a beating... its nice to be able to rely on some decent bottom when you don't feel like riding 9000rpm all day. I know how to open my thorttle, very well. But I like power opening it at the bottom too, not just up top.
Its funny you say a bike with bottom won't be as fast as a bike with more top. An '01 YZ125 is pretty much a topend screamer, and a '99 KDX220 is all low mid, nadda on top. Gearing being the same, not meaning the same sprockets, but for the same top speed topped out in 6th, these bikes were almost completely side by side all the way from a standstill to topped out. The YZ was a little quicker, but not by very much at all. Very very little. So whats the point... well, obviously bottom end counts for quite a bit, because the KDX220 doesn't have nearly the power peak of a 125... but it pulls hard and consistent at its optimum RPM range, and has TONS more torque than that 125 could dream of. So you have a bike thats got a much more rideable powerband, and about the same avalible acceleration. Also, you can pull quite a bit more gear on the 220 than on the 125, and still be able to ride much easier in the woods. Ask anybody who drag races, HP is nice, but torque wins races.
Not getting personal though... I love my 125 and don't mind fighting to stay on the pipe. But bottom end is very important depending on the application. Good food for thought anyways.