Are ticket barriers worth having?

If you MUST have them, what considerations do you need to take into account? I think there are two main areas - physical and ticketing.

Physical

The kind of full-height interlocking meshing turnstile which you see on the New York Subway (and which I associate with sports grounds) are unfriendly, unwelcoming and very difficult for passengers with luggage.

The old Eurostar barriers (now replaced) where you inserted your ticket, watched in bemusement as it shot in and out two or three times and then retrieved it from the same slot, are slow and confusing.

It's much more intuitive to insert a ticket in one slot and retrieve it from another one a step onwards in the direction of travel: this speeds up passenger flow as well.

The systems which have two retracting gates provide them in different forms.
Eurostar in Brussels has 2-metre high glass ones. These are confusing until used by someone who knows how they work - then herd instinct takes over and everyone follows the crowd.
London Underground has two flap-style opening gates: these seem to work well. The extra wide ones, for wheelchairs and buggies, are valuable but of course can allow more than one person through at the same time (although frequent users of the Paris Metro will tell you that no barrier system is proof against this!).

The three-legged milkstool arrangement is also good, especially since passengers with wheeled cases can drag them after them underneath the closed gates.

Ticketing

A major defect of barriers is that they do not allow ticket sales on board - something which is popular, even if such tickets are sold at a premium.

I remember the Gatwick Express posters showing a train seat labelled, "Our latest ticket office". The clear message was that you'd queued at check-in, you'd queued at security, you'd queued for a coffee, you'd queued for duty free, you'd queued to get on and off the plane and you'd queued for Immigration - but you didn't need to queue for a train ticket! Wonderful!

I know of no way round this.

Maybe promoting bulk advance purchases (pre-pay, pre-purchase, carnets) is an answer. It would be one way of reducing the amount of queueing passengers have to do. Maybe doing so in conjunction with some kind of Fast Track package would help sell the idea.

If you must have barriers, ensure that they are compatible with 2D barcodes on mobile phones or paper printed-at-home tickets. Ensure that they are compatible with encrypted numeric codes (for people whose mobile phones don't support pictures).
One brand, on trial in Sweden, claims a processing time of 50 milliseconds. This might lead to an always-open barrier - one which is open as long as people are presenting ticekts to a reader: if someone doesn't, the barrier closes until they do.

Barriers need to be staffed. Staff need to be aware of potential problems - prams, buggies, luggage - and need to be pro-active in handling them. Training - ideally by giving the staff the problems themselves to surmount - is vital.

Are barriers really a good idea on airport railways?