Mainly Audible as I recall (it's mostly used by my GF, not me, and she has or had the subscription to Audible), but it's an issue for encodes as well if the tracks are large. Some (good, IMHO) audiobooks on CD have tracks every few minutes. This means jumping forward if you have to start over from the beginning is rather easy, just skip tracks. If the entire CD is one (or sometimes two or three) tracks, then you have 30-70 minutes of audio in one block, which is unwieldy when you have no time elapsed counter.

By the way, for frequent audiobook listening, buying them through the iTunes store is a rip-off. Much better to belong to Audible and get two books a month for $20 or whatever the rate is these days. Get quite the money's worth with that compared to buying individually.