The problem is that too-high fuel pressure is possible. If the fuel pressure is above about 3.5 PSI, fuel can be pushed past the float valve and cause flooding. If you are using a mechanical pump, there is no easy way to adjust the fuel pressure, so I think that's why they recommend a regulator. Three problems with that, though: first, many regulators are trash; second, many won't work well with a mechanical pump, and third, the regulator may not work well with a small difference between the input and output pressures.
I use an electric pump that puts out about 6-7 PSI and a Holley regulator to bring it down to about 2 PSI, which is plenty. I also have a pressure gauge permanently connected to the regulator. That's not necessary, of course, but since I've had fuel-pressure problems in the past, I like being able to check the pressure at a glance.
If I had to reduce the fuel pressure with a mechanical pump, I think I'd be tempted to try trimming the spring. I've never tried that, though, so the suggestion might be nonsense.