I think both types have there place. I personally deal 100% with "repurposed" materials (scrap). It isn't meant for what I use it for, but it's free. It's not clean metal of a know type; it's rusty, bent, ect. So advice from pros is sometimes useful, but often there advice is difficult to apply in the field. I can safely say i had no idea what a 6011 rod was when i first bought a 5# box, and reading posts of actual pros explained a lot.
Also there are different pros. One guy might be a heavy equipment service guy who works 10 miles down a mud logging trail; while another is a production mig welder at an aluminum awning maker.
There is also different types of advice; for example: let's say preheating cast iron: one guy says preheat to 500 degrees for x minutes; another say heat it till you can spit on it and the spit evaporates by the count of 3.... guy 1 might be more right; but guy 2 was more useful in the real world...
If I'm on a road project and I need a grader rigged up to plow out a 3 foot wide, 16" deep box to widen the road, who would you ask, the equipment guy, the pipeline guy, or an aerospace tig guy, or the home owner who welded up a lawn mower deck? I'll take guy 1. Now, I've got a magnesium transfer case with pump rub holes, who do I ask? I'll take the aerospace tig guy. I've got a 8 ft deep mud hole with a 4" steel gas line leaking next to a busy state Road and a interstate ramp (2 weeks ago...).... I'll take the pipe line guy. I've got a street sign post and rebar plow with cut up mower blades as the plow edge; and I want to add on to it; who do I ask; **** if I know; you just read through a jumbled up fight between the 4 pros and the guy who fixed is mower once...