What is your specific use case? I used to work managing API's for one of the GDS companies
For screen scraping, there are a number of companies that do this already for multiple carriers. I'd suggest making use of one of these rather than reinventing the wheel. Example: http://xmldocs.travelfusion.com
My project definitely isn't applicable because it requires caching prices, I don't remember why we were declined access for our other teams/products though. I would have to ask them, as it's been a while.
About screen scraping, I guess it depends on how quick you are at writing scraping scripts (and if you find it fun). I could probably write one faster than I could register for that API. You're also adding a layer between you and the prices. You don't know how long Travelfusion has cached these prices for, while it's simple to write a script that opens the real booking engine to get correct prices. I typically only run mine when I'm ready to purchase a ticket, but don't know which dates, rather than running it constantly to find deals.
And if I were doing anything more than a few routes on a few carriers, I definitely would use a real API instead of reinventing the wheel. In this case I just think it's kinda fun.
For screen scraping, there are a number of companies that do this already for multiple carriers. I'd suggest making use of one of these rather than reinventing the wheel. Example: http://xmldocs.travelfusion.com