The least expensive single-sided meters tend to be left crank arms and pedals, so I guess the appeal would be ease of install and portability. With the crankarm, you could easily cover multiple bikes as long as you were running the same type/length of crankset (all 172.5mm Hollowgrams, for instance) on each bike. The pedals, on the other hand, are damn near universal. Even though I have a definite preference for Speedplays, the $800 Powertap pedals were very tempting from that perspective - in under two minutes they could be moved to any bike in the house, including the wife's Dutchie bike with wicker handlebar basket.