I think that those hosts who say they are fine with Instant Book and try to convince others that's it's really okay, may not understand why many of us won't use it. It's partly a matter of some of us not wanting anyone in our home who we haven't communicated with prior to booking. But it's also a matter of what you offer. @Robin4 touched on it above with the comments about why guests are booking. But also, if your listing is in some way out-of-the-ordinary (I know we'd all like to think of our places as special, and all are unique, but that's not what I'm referring to), IB has more of a potential to be a disaster.
Most guests, for instance, would take Wifi connection to be a given. What, I can't watch Netflix? If you live somewhere where that's not the case, guests could arrive, assuming that there's constant, unlimited internet (yes, even if you stated clearly in your listing that there wasn't) and be upset about this, demand a refund, etc.
Many hosts live in areas where the majority of guests would be arriving by car. So if it's a 10 minute drive to the nearest big supermarket, it's no big deal. If most of your guests fly in, take the bus, etc, and don't have transportation, and it's a 20 minute walk into town, hosts need to make sure that guests are clear on this before booking.
Living in the tropics, I can often ascertain through pre-booking messages whether the guest has traveled widely, has been to this area before, or is just some city slicker who thinks coming to the beach in Mexico is going to be just like the magazine pictures and will have a major freak-out if they see a spider, an ant or a cockroach, which are endemic to the area and have nothing to do with whether you keep a clean home. It wouldn't make me decline the booking, but I'll have a better idea of how I might have to present such possibilities to a guest when they arrive. A guest who tells me they come here every year or two, or did a two year stint in a mud hut village in the jungle somewhere, I know I won't have to stress to them that they shouldn't leave food or food wrappers lying about their bedroom.
I know that even hosts with listings that are in quite civilized places that have all the amenities and bells and whistles that one would expect in a city or suburb have IB guests arrive who haven't thoroughly read the listing, arrive thinking they're getting an entire house, when it's a private room in a shared home, among other misconceptions. So you can just imagine how much more of this hosts who have off-the-beaten-track listings would be dealing with if we used IB.
And, in my case, as a single female hosting a private room in my home for 1 guest only, about 70% of my guests tend to be women travelling on their own, (I accept male guests and the ones I've had have been great) usually between the ages of 30-60. I've found this demographic to be wonderful guests and they are not the least bit interested in Instant Booking. They like to communicate with the host before committing to a booking, it makes them more comfortable to have some sense of the host whose home they are thinking of staying in.
