Originally Posted by CPRich
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If the airlines can negotiate with one GDS to sell tickets for a $6 fee, and others stay at $12, they'll drop the others. This would make their flights not available to those using those other systems, unless they pay to subscribe to multiple GDS's - quite expensive.
Not to mention all the GDS systems use different formats so it's not as easy as it might seem, this is not a 'windows gui' world by any stretch of the imagination...