It has a relatively big effect, but depending on your personal looking at it, the advantages can be also be relatively unimportant. As Merentha said, the main thing is not the higher maximum, but the speed you increase you proficiencies through fighting. The higher the difference between the maximum (given of WM) and your actual proficiency, the faster it will increase. So it makes sense to increase WM at the beginning, if you increase it at all.
I played some chars with WM 10 and some with only 2, and I must admit, the faster increase of proficiencies is significant. I had a level 25 char with an his main weapon prof high above 400, and another char at level 30 had about 250 at his main skill. So the difference is really big, but the reason why many people say it's relatively useless is the fact, that the weapon prof isn't that important after you reach a certain level. It matters only at the beginning. I tried it out with a melee character and I really didn't have the feeling that 8 more points in WM are significantly helpful. Sure it gives a nice bonus (higher prof is always better), but it's usually not worth the 8 skillpoints you'll have to invest. Especially at higher levels you will fight for every single point, so if you don't want to increase WM for style (as I do), I suggest leaving it to 2-3.