@Pete69
My experience is that a great number of guests use IB. I switched it on at the beginning of this year, not because I wanted to, but because I had to. Despite Superhost status, lots of glowing reviews, a competitive price etc. my listings were all but invisible without it.
Since then, all of my listings get booked back to back and the vast majority of guests use IB to do so. The ones who request book are usually first time users, who can't IB my listings because I require a positive review for that.
I only occasionally get a guest who sends an enquiry instead, usually because they have a question. Most guests don't need to send any questions before booking as my listing descriptions are very detailed.
To give you an example, for August I have 22 bookings so far and 18 of them are IBs.
As for the desirability of IB guests v Request guests, this was something I was really worried about when I started using IB, but I don't really see any difference. You get good ones, okay ones and bad ones in both categories and it is not always possible to spot the bad ones at enquiry stage.
The advantage with IB is that you don't get penalised for not responding to a booking request within 24 hours, or for declining a request. More importantly, you can cancel penalty free under certain circumstances. The penalties for cancelling when NOT using IB, on the other hand, are very harsh.
I'm not trying to do a sales pitch for Airbnb. I was actually very resentful of needing to turn on IB, but I have to admit it's worked out fine for me.
The glitches described on this thread need to be sorted out though.