Although it's more expensive to do a separate pilot, it's not always a waste of time or money. In many cases, ideas that on paper are very good don't translate well. A pilot lets you catch these problems and remedy them. If you go straight to series you may end up only finding out about these issues when it's too late to fix them.

A few good examples of this: the (unaired) pilot for BBC's Sherlock is a not fantastic 60 minutes with significant pacing problems, which was subsequently reshot and turned into a well received 90 minute miniseries. The US version of Life on Mars got completed re-located and re-cast as the pilot was just bad (...most people wouldn't call what ended up as 'good', but it was a vast improvement on the pilot).

Neither approach is without its problems. Take a show like Game of Thrones, which is very expensive to make. On the one hand, HBO could have saved millions of dollars by not ordering a pilot. On the other, it was pretty clear once it came back from post production that the approach the writers had originally taken wasn't going to work, and a number of changes were made to make the show more accessible to people who hadn't read the books.

So HBO did 'waste' several million dollars on a pilot that never aired, but the ability for the creative team to review the pilot before embarking on a full series commitment meant many changes could be made that resulted in a better show. HBO have easily recouped the lost money.

It's interesting that Amazon have taken the opposite approach to Netflix, and ordered a large number of pilots rather than going straight into series commitments.