This is a good reminder that PPC numbers need context. Conversion rate is useful because it tells you how often visitors take a tracked action, but it doesn't tell the entire story.
You also need to know what those conversions are worth, how much you paid to generate them, whether the leads are qualified, and how many eventually become paying customers.
A campaign with a modest conversion rate can be very successful if it produces high-value customers at a reasonable acquisition cost. On the other hand, a campaign with an impressive percentage can still lose money if the conversions aren't valuable.
So I agree that benchmarks should be used as a reference, not as the final goal.