What if the fields contain tabs? For every human-readable format that can contain arbitrary user input (nearly all of them) you need some form of escaping (I guess you could do length prefixing in ASCII decimal, but it'd not be pretty either and incompatible with basic tools).

But by far the biggest problem is the logging of text messages aimed at a humans, not the delimiting. Regular expressions can help in searching logs in quick-hack jobs, but if you need to parse logs for visualization or reporting, which is very common in organizations, using them is error-prone. After all, you rely on English messages of a certain form. The complexity of that could quickly move from "easy with regexps" to "we need NLP in our log parser!" (never mind security problems with one field leaking into another due to a slightly wrong regexps).

The application might change the message to make the message more readable for humans, or even move around fields, and your automated parser breaks. Structured messages, on the other hand, won't change for those concerns as the formatting for humans happens in the back-end.

I get a bit annoyed at the "WTF kid, learn UNIX tools" kind of responses here. UNIX tools are one way of doing things, not the holy one perfect way. Tool support is important, but there are also tools for processing JSON, XML, streams available. Shouldn't you use the best tool for the job, not the one you happen to know?

(I don't mean that JSON is necessarily the best format in every use-case, but for automated processing every structured format at all trumps arbitrarily delimited and escaped files. You can easily convert between structured formats should the need arise)