As if there weren't already enough reasons not to invest in legacy airlines.

On Wednesday, UAL Corp.'s (NASDAQ:UAUA) United Airlines suffered a computer glitch that knocked out operations for two hours. Naturally, the results were devastating. Twenty-four flights were cancelled and 268 were delayed, most by at least 90 minutes.

You know what that means. Cranky employees harassed by justifiably cranky customers. Here's one, quoted in an article run by the Chicago Tribune: "Not all those computers are manned. I don't think they are doing their best."

Oscar the Grouch would be thrilled. Investors, not so much. Airlines already score miserably when it comes to service, United more than most. The last thing it can afford is for fliers to believe that its employees don't care.

I'm sure most workers do. But I'm not as sure there's anything they can do to persuade customers to change their minds. At least, nothing short of the multimillion-dollar customer's bill of rights JetBlue (NASDAQ:JBLU) offered to its stranded masses when blizzards incapacitated its fleet in February.

United doesn't have the money for that. Nor do legacy peers Delta (NYSE:DAL), Northwest (NYSE:NWA), and AMR Corp.'s (NYSE:AMR) American.

So, they'll muddle through, as they always do. Customers, meanwhile, will keep flying as cheaply as they can, confident that when it comes to airlines, reliable service is a luxury they shouldn't expect to receive.

Beware, investors.

Do you agree? Disagree? Let me know.

Fool contributor Tim Beyers still flies United as often as he can. But he also thinks it's a very different carrier from the one he grew up with. Tim didn't own shares in any of the companies mentioned in this article at the time of publication. JetBlue is a Stock Advisor recommendation. The Motley Fool's disclosure policy prefers the train.