I use Y for many of these: washer and dryer, hair dryer, dryer than dust, frequent flyers, a flyer about a concert ... It bugs me when, for example, crossword puzzles use the other form and don't mark it a variant.
To me that's intuitively easier to read and pronounce than with -ier, which is more common in other phonetic contexts: easier, happier, bubblier, cashier, croupier ... It just feels different from dried, tried, spied; somehow if we wrote those with a Y, the suffix would feel more like a separate syllable, more like dryad or triad. Which is why it works with -er, because that is a separate syllable, unlike -ed in these cases.
We all spell pliers with IE, but in AE at least, we tend to pronounce it more as one syllable than two, like fires or wires. Etymologically they are a tool used to ply, I suppose, but nowadays I would think of a plyer as more two syllables than one, and as a person who perhaps plied a craft, whether a boat or a trade.
I'm familiar with drily as the older spelling, but somehow it looks old-fashioned to me; I would expect dryly in a modern book, at least in AE.
That may not all be 100% logically consistent in the view of dictionary editors, but it makes sense to me. (-: