Well shapers are certainly obsolete in industry, they are too slow. For amateurs…well, we are not usually concerned about time so much, and cheap tooling can make up for any extra time taken. If you have the space a shaper can be handy thing to have. If you don't have a lot of space, a lathe and a vertical mill is probably a more versatile setup than a lathe and a shaper. But you won't get the finish that a shaper can give you from a vertical mill, unless you can afford a good CNC machine, and that will be rather more expensive than any shaper would be.
As far as the dovetails go, well, so far I have needed to do dovetails with 55 degree and 60 degree angles, so that would double the number of cutters needed. Wouldn't matter for production, where you are probably going to wear them out anyway, but for a one off it becomes a bit more questionable. I've also done T slots, although for some of those I do have the T slot cutter to fit the mill. They are a bit faster and less fiddly to do on a mill, but the shaper uses home made cutters which are certainly cheaper.
When I find that the nearest piece of scrap is a bit oversize for the job needed…well, there is nothing like a shaper for ripping a bit of stock down to size.
Shapers are good for work on stationary and marine type steam engines, where a lot of the work on the original would have been done on a planer. There is probably not so much work in a loco that a shaper would be useful for.
But after all, it is a hobby, I enjoy my old shapers. If you don't happen to like them, feel free to use something else. Someone local has a $50,000 CNC machine at home…I am sure I would enjoy that too, if I had that sort of money to throw around.
John