I actually must be one of the rare few who belongs to another school of thought on this subject. I own a couple of bolt on options, including the very well designed and expertly manufactured units sold by Torqueboards. I prefer the welded option for the following reasons.
1. Strength and security. This is not to insinuate that the bolt on options are weak, they are not. At the end of the day though my experience with the bolt on options is that they "can" and do move, if you ride them hard. There are tricks and tips for installation which can dramatically reduce their susceptibility to movement, but I have yet to find a combination that 100% prevents movement. For full disclosure i'm using Paris trucks, which makes this issue more profound. To further complicate matters I also run a dual diagonal configuration, which means my front truck has a mount attached to it and that truck is more susceptible to downward force when the board is "dropped."
2. Perception versus reality of "adjustability." Do you know when the adjustable factor of the bolt-on mount is most useful to me? Predictably its when I need to re-adjust it following it unexpectedly moving from its position after a hard ride. Fact is that once I had selected my trucks, wheels, and drive gear the need for adjustability in a mount was overvalued. If you don't ever change your setup, why do you ever need to adjust the mount? The simple answer is, you don't. So once you've found that combination that you love and will ride for the forseeable future, why introduce insecurity to it?
3. Simplicity and cleanliness. The welded mount in addition to the peace of mind of knowing that its NEVER going to move, also brings the added benefits of a simple and clean installation. The mount itself is smaller in mass and less bulky and complicated looking. This creates a clean and simple finished look. As an added bonus the space savings make it easier to accommodate during build design, meaning it will work with a much wider variety of trucks.
Lastly COST. Lets not underestimate this one, but lets also tie this back to the the original argument that supporters of bolt on mounts consider to be the bolt-ons largest single advantage.... adjustability. The cost of the bolt on mount is generally twice that of the welded mount (minus paying someone to weld it if you're unable to do so yourself). So the bolt on supporter will say "yeah, you save money but you can never move it which means you can't change wheels or belt widths once you've set it," ..... and that's generally true. But we are DIY'ers right? We want to fiddle, and change things, and experiment with different setups. Hangars are cheap, and if you want to experiment with different truck types you'll need different bolt on options anyway to accommodate different shapes. So if I want different setups to accommodate different wheels, for the same cost of a single bolt on mount I can have stronger welded mounts on 2 different sets of hangars.
I dunno. I don't see much advantage to the bolt on options. Overrated IMO.