Pounds Mass vs. Force
I know people who use slugs fairly often, but I personally use the other unit I mentioned, the lb-s^2/in, really quite often in the work that I do.
The folks that I know who use slugs usually just don't know about, or don't feel comfortable working with the lb-s^2/in unit, but it works just fine. When all your dimensions are in inches, as they usually are in machine calculations, this is the logical choice.
There really are not any other good choices as long as we stay with US customary units. If the US would ever switch to SI, then we could move to kilograms and things would be easier. Frankly, I don't think I'm going to live that long, however.