I was talking about "holding" deposits, sometimes bundled up with referencing fees. The holding bit is meant to be a downpayment on rent/ deposit (the deposit held against damage, that is).
The trouble with the holding deposit is that if you step away (e.g. the letting agents/ landlords are dragging their feet and another, better offer comes up for you, for example rent-, location-or time-wise- you've run out of time to move), you won't get it refunded. There's no bond on the other side, so if the LL decides not to let, they lose nothing.
You may think "this wouldn't happen" to you, but we once had this happen, and once were in danger of it, over 3-bed Victorian houses, with rent of £1,000+ p.c.m. - not exactly a vulnerable demographic! The letting agents in the first case were horrified, but still kept the "referencing" portion of our "holding deposit". 
Just make sure you're renting in an area with plenty of competition. 