Per a recent Dr. Dobbs article, the only reason to write unit tests is as a “convenience … to track down bugs faster.” If that’s the only value proposition, then I must agree with the author that writing unit tests is a “luxury” that only infrequently makes sense.
You’d be far better off investing the time in improving your integration tests or (much better) bolstering the tests that demonstrate customer acceptance criteria. They run much more slowly, but provide more value than writing unit tests after you code: They not only help prevent you from creating defects in the system, they can also be a key negotiation point, and they can act as living documentation on the behaviors your system supports.
Fortunately, writing tests first can create myriad other benefits that outweigh their cost: reduced system size, tests that improve developer understanding of system behaviors, more decoupled/cohesive designs, cleaner code, and the ability to change code safely and rapidly as it needs to be changed. Yes, there’s some churn along the way, but it’s usually mild (and milder as the quality of design improves). These benefits are why I’ve done TDD for a dozen years now, having witnessed some fantastic successes along the way.