Somebody on this forum suggested that they where going to use it. I haven't seen such personally. At work almost all linear rails sit on tooling plate that is then bolted to the T-Slotted extrusion. This is done mainly because it is far easier to get the tooling plate drilled precisely on a CNC than it is to hand assemble a T-slotted extrusion. If done right (not always the case) you can have everything drilled and tapped in one go. At the moment I don't know of any linear rails (at work) being bolted directly to T-slotted extrusions.
The other option is to use a welded steel frame, with welded pads on the frame work, for your linear rails. The big problem with this approach is that anything of size needs a well equipped machine shop as the pads are usually milled flat and coplanar. Thankful there are work rounds for linear rail mountings on steel tube for the DIY world. The biggest reason I can see for going steel for the frames is that it is actually cheaper to weld one up and machine it, especially if you are looking for reliability. The biggest problem with T-slotted extrusions in my mind is that they do not stay together for the duration unless you get really excessive with gusseting and tie plates.
A side note:
Mush of the extrusions we use at work are for special machine application or sometimes called automation. One of the reasons I call it T-slotted above is because we have all sorts coming into he plant over the years. This includes stuff from 80/20, Misumi, Bosch, Fastek, Some German company (don't have a name) and a bunch of others. For whatever reason our engineering department doesn't seem to care though it does cause a lot of grief for the rest of us. Right now I'd have to say that we are roughly 60% 80/20 style and maybe 30% Bosch. That unnamed German company (best guess to be German) actually makes a really nice extrusion with heavy walls and flat surfaces.
Not much of this is used in the primary structures of the machines though.
I'm not sure where this idea that working with steel is so bad got started. Ee are talking mild steel here or structural steels which beyond an occasional hard spot isn't that difficult to drill. Generally I find steel tubing to be rather easy to work with even compared to rolled structural steel. I'm not sure if that has to do with being formed from sheetmetal stock or what but it is generally easy to drill, cut and form. In other words we aren't working with tool steels or work hardening steels. The biggest problem a person might run into is designs that call for a milling machine, the way around that is to design to minimize the need for such machines.
Cutting steel isn't too hard, you can do it with an angle grinder, but it is much easier if you have access to a large bandsaw. Getting things flat is where steel can be tricky.
With practice an hand drill, possibly with some guides can be passable but yeah a drill press is a smart choice. The good thing here is that you can get by with a pretty pathetic drill press if you can support the work, this mostly because you will be drilling small holes for tapping.Not too hard to drill and tap some holes, but as a minimum you need a good variable speed drill press. A simple hand drill isn't good enough. I use a small mill.
The biggest problems with such extrusions is the costs. This especially when I see drops for so little at the steel supplier. The difference can often cover the cost of a good right angle grinder and a drill press and probably a few more tools.The other option is to use aluminum extrusion, and I'm not talking about T-slot. I'm talking about square aluminum tube, perhaps 3/8" wall thickness, not to hard to drill and tap some holes for your rails, and you could fill it with something afterwards like high density polyurethane or epoxy granite, you could even press in so ribs made of plywood. IMO, performance wise, that would be superior to T-Slot, and cost wise it might be on par. You'd have to make some inquiries to see what is available, and visit the metal merchant with a long straight edge to see if it's flat. IMO, aluminum products are typically flat enough for our purposes.
I might add that it pays to make up a few good tap guides. A broken tap can be more frustrating than anything that would happen at the drill press. If nothing else somebody not familiar with metal working probably should practice on some scrap metal first. At the drill press using techniques form the wood rocking world, such as fences and hold downs can lead to excellence repeatability in one direction. so your layout lines only need to be placed for one axis.If you do this, please report back, I'd like to hear the results for cost and flatness. Also, look at the price for a full length from the wholesaler, and you'd use the rest for other parts of the machine, otherwise the cost may be double for what you get.
Aligning the rails and drilling / tapping some holes is really a joke - not hard to do, but like I said, you need at least a good variable speed drill press.
I think part of our goal is to get people to relax about steel tubing. Yes it is a bit harder than aluminum but it isn't unobtainium. Steel tubing might not be the right choice for everybody and every situation but it does have its place at the DIY table. Also unless a person wants a light weight transportable machine, weight isn't a bad thing in a router design.