Even if this is true, the problem is you don't know what's going to take an afternoon of programming. How many times have we all said "should only take an hour"

Even if this is true, the problem is you don't know what's going to take an afternoon of programming. How many times have we all said "should only take an hour" just for it to take four days? If someone spent the time and effort building something you need, I don't see a problem with using it. It all depends on what kind of system you're building, what the security and stability guarantees are, and in general what trade-offs you want to make. In other words, "never" is usually bad advice.