Well, it can be really useful for clients. But, the book itself, is it just a formula with cmyk equivalents or is it one which has the real pantone swatch on the left and the cmyk swatch/formula on the right? Those are great because you see exactly how it 'should look' and how it will look.
Other thing to consider though the age of the used guide. Was it kept out of light? Indoors? Those books fade after a few years, especially if they were kept spread open on a light table somewhere in the corner or so.
Also depends on the complexity of the job. Nowadays, even at an offset litho printer printing 4C is as cheap as going 2 color. Depending on who you use, how 'lazy' they are, what they may have source out..etc printing a PMS spec'd job as process may actually be cheaper.
I've run across that many times. So, in that case I could show the cmyk equivalent of a pms color and let them see the difference.
But, if you're printing true duotones or have to use a client's pms color, then a formula guide is only a second choice to an actual swatch book.