[LazyMan]

Oops forgot the image.

[LeeWay]

I think it all depends on the design itself. These extrusions are capable of great strength. This is especially so when coupled with steel at various points.
I have built an accurate mill using 80/20 as a base and column. An accurate lathe as well. They are the basic structure that steel was then added to for mounting linear rails and screws. The ease of assembly was a plus over going with something else.
In the case of the mill, it was likely cheaper than steel weldments would have been once all was said and done.

That said, steel weldments can be a good machine too. So can bolted steel joints.
It all depends on the design, size, and profiles chosen.

I always make an extra effort to include adjustment features as much as I can. This has always proven fruitful in the final alignments stages.

To sum up, there is no wrong choice between the two, just different design requirements for different types of machines.