Rapid Shots do more damage than Power Shot. The belief that PS is your filler attack is a fallacy I see from a lot of Merc dps. This comes from the fact that most Merc dps are PvE oriented. There PS is your filler. But not in PvP. Unlike in PvE, you need to account for the reality that only about 40-60% of your PS attempts will complete to do advantageous damage to your target. The rest of the time, the target moves out of range/LoS, is killed by someone else, is cc'ed by someone else, you get interrupted, etc. Using PS in PvP is an incredibly inefficient heat/actual damage strategy. In contrast using RapidShot towards the front end of your rotation is incredibly efficient from a heal/damage ratio perspective because of CGC. If you are properly using RapidShot once every 6 seconds, it actually becomes your second most powerful attack after TD - even more powerful than RailShot (I am not counting FM since that isn't generally part of a rotation sequence).
I've said this before, but of all the matches where I have scored 700+k or 800+k, I don't think I have ever used PowerShot. Not once. Way too busy in those kinds of matches to spend 1.5 sec on something that half the time fails to even fire.
Its also a fallacy to assume everyone screws up on Power Shots 40-60% of the time. I find my own Power Shots tear into people. If all your other attacks are on cooldown, you can't really help burst anyone down with Rapid Shots. But if the situation is right, you can chain cast Power Shots for a huge amount of damage.