The English guidebooks tend to be out of date--written at a time when, I believe, pilgrim accommodation often involved sleeping on the floor.
That said, spring is getting busier on the Via de la Plata. There were times I'd have felt more comfortable with a mat as backup because albergue beds did sometimes get full. (I walked this April/May, and apparently June was also very busy.)
Of course, I don't know if I'd take a sleeping mat if I did the route again in spring--I never absolutely needed it, though I did spend one night on a floor. And from what I've read, I definitely wouldn't take one at any other time of year.
Anna-Marie