The no turn-away policy is the key element of hike/bike sites for me. In California the official policy says that even parks with hike/bike sites can turn away cyclists and hikers if they are full, but in practice it isn't done and the rangers will find some space, such as a picnic area, for the extra cyclists to use with the condition that they take down their tents fairly early in the morning. My experience at other parks has been mixed - while I've always found a place to camp the ranger attitude has varied from friendly to quite negative. E.g. once I was offered a site in a loop that was nominally closed for maintenance but another time I was just given directions to another park about 40 miles away (I ended up finding another camper who was willing to share their site and the cost). California State Park camping areas tend to fill up very early - frequently already a month or more in advance with people making reservations as soon as they become available. So having some no-reservation/no turn away sites is reassuring when planning a cycling trip where sticking to a day-by-day itinerary is difficult due to unforeseen weather, minor ailments, etc. OTOH, in my tour in Canada last summer (Jasper - Banff - Montana) the availability of campsites was much better.

The other big positive of the hike/bike sites is that they encourage interactions with other cyclists. Especially on popular routes like the Pacific Coast there will be other cycle tourers with whom to share experiences.