There are really good rims that are pinned and not welded. Back in the 70's the sidewalls of rims were pretty narrow, but brake blocks were very narrow as well. As equipment manufacturers tried to get better braking, one way they did it was to go for bigger brake blocks. You can only go so much longer before you actually have to curve them to fit the radius of the rim (a la some MTB brake blocks) but you can make the braking surfaces wider and use a taller brake shoe with very good effect. That's basically what happened.
The welding was actually a more cost-efficient way to assemble a rim. It creates a point in the rim where if you're lucky you have a smooth continuous surface or, if you aren't, you have a slight dip or bobble. As a result, many top wheelbuilders prefer pinned rims (look at the number of top builders who like to recommend Velocity rims, or who love to use old vintage Mavic tubular rims, for example).
As for the machining itself, it was partly a way to address a hassle with welding, but also in the search for taller sidewalls to improve braking, it became hard to make an extrusion that had reasonably flat surfaces. On older non-machined rims you'll notice that the braking surfaces are rarely flat, and when you go to a bigger braking surface, your rim cross-section either has to have very thick walls (and thus be quite heavy) or it starts to have the cross section of a burrito and has to be machined. Flat grinding the sidewalls of a rim is inexpensive to do -- it's done on both sides at once and in a fixture that ensures everything is ground in a flat plane. A byproduct is that it makes a rim that starts out basically reasonably true; it also takes some weight off in an area where the extrusion die can't control wall thickness as well.
As for Deep V's, they use rather thick wall thicknesses so even the non-machined rims aren't too irregular at the braking surfaces. If the rim is anodized, there's a very slight difference in braking, but remember that many rims have traditionally been anodized anyway and the anodizing on Velocity's isn't a deep hardening anodization. The gold rims are anodized, as are all of the tubular rims (because tubular rim cement doesn't stick reliably to powdercoating and you can't powdercoat the sidewalls and not powdercoat the rim bed as well). Powdercoated rims will have more flaky braking (pardon the pun) as long as there's powdercoating left on the braking surface and predictably when your hot red powdercoating is half worn off, it'll look messy.