How many users would these services have lost? Some users will be dissatisfied with the services so they would lose potential customers. Also, another competitor could see that they could do better and launch their own service, diluting the market share for these comapnies. But these companies sacrificed revenue by not releasing their service until it was very strong. Also, there are other companies that took years of work and flopped miserably upon release. Farecast and Pandora faced the risk of joining that unenviable pool.
It's hard to argue that Farecast and Pandora went down the wrong path since they were super-successful startups. But if I'm not mistaken, both started in academia so they have a higher standard to live up to in terms of rigor. Most startups are under the pressure to launch something quickly. If you were running a similar company, when would you launch?