I suspect it rather depends on where the property is and the state of the market, I have bought and sold a number of properties over the years in a number of locations and generally viewings are a free for all, however as and when offers are made details are provided regarding the position of the buyer (are they a cash buyer/need to get a mortgage or are they in a chain for instance), but its hardly an onerous process which any agent good or bad should be entirely capable of doing.
Heard too many horror stories regarding Purple Bricks etc, they depend on fast turnover (hence the low fees) and if any property sticks they seem to lose interest after a few viewings and basically leave the vendor to it.
As noted previously on the whole you are paying 1.5% to get the property onto rightmove etc, and possibly to access the agents database, most agents from experience know very little about the market, even domestic valuations is not the rocket science many would have you believe.
Good post and it makes a lot of sense what you have written. Purple bricks seems to get very mixed reviews depending on who you talk to as many say once they have got your money and the initial interest dies down they dont seem so enthusiastic.