Call me OCD, but I actually take this very seriously when problems go up on my bouldering walls. Double-taping is a no-no 99% of the time, with exceptions. Here are some of my points and reasoning:
1. DOUBLE TAPING = LAZY SETTING. Unless your gym/wall is strapped for holds (and professionally I think there's a much bigger problem if you're THAT tight on holds that you'd have to double tape every hold on your wall), double taping is bad work ethic. Good setting means that every hold, whether it's a hand or foot hold, should be set according to your route. Even foot chips should be directional and specific to your set. If you re-use another hold, chances are that hold is going to be off in angle, orientation, etc. Most of the time, double-taping is due to the setter being lazy. They can argue that it's the only bolt that works with his beta, but you can always work around by adjusting the beta and moving the holds just a little bit. Honestly there's really no excuse.
The ONLY time I allow double taping is on giant holds and volumes to better utilize the available space on the wall. For example, the Rockwerx volumes (Boob, Growth, Spore) and some Motavation volumes.
2. TAPING IS LIKE PUTTING THE SHINY CHERRY ON TOP. In a professional sense, even if you set a great route/problem, it's not going to be as apparent and climbed on if your customers' eyes aren't attracted by it. 9 out of 10 times, people are attracted to routes by pure aesthetics. There's no denying this. Ever notice that routes with giant, heavily features holds have more climbers on it then ones with smaller, older-looking holds, regardless of how well it's set? A well-taped route is one effective way you can attract climbers to your climb. It's like.. putting the shiny frame around your painting. Would you sell to your clients your painting on a flimsy piece of paper, or would you put it in a solid frame and attract more customers to it?
3. TAPING AT CRAZY ANGLES. Most climbers don't have tunnel vision. Climbers will be able to see which hold goes to which climb without the tape being at crazy angles. Whenever I see a setter put tapes at ridiculous angles, like totally horizontal or even straight up.. I mean really? At those angles, it's just asking for shoes to rip them off the walls. A rule of thumb I tell my setters to use is, from the tape being at a 90 degree, straight down position, you don't need to angle the tape more than 45 degrees in either direction.
Now some people will argue that "Oh, well, tape still gets ripped off anyways even if I angle them straight down." If you tape everything correctly (read point #4), aside from flopping feet of new climbers, the only time I see tape being ripped off soon after it's set is if.. are you ready?.. it's double-taped on a hold (another reason why double-taping is bad).
4. TAPE NEEDS LOVE TOO. Brush your damn tape. No seriously, do it. The glue on the back of the tape isn't super glue. You need to actually put your thumb, or even better a brush, and brush/smear the tape that you put on, especially if your walls are textured. Just slapping on a piece of tape and hope that it'll hold its own to the countless hands and feet that'll graze by it is just ignorant.
Try this little experiment. Put two pieces of tape of the same color next to each other. Brush/smear one of them and compared the two. Do you notice a difference?
5. ROUTE TAPE IS LIKE YOUR UNDERWEAR. Sometimes no matter how well you place them, it's going to be at the mercy of a badly placed foot, and needs to be changed. If you want your gym to look and be as professional as possible, the walls need to LOOK the part. That means globs of mangled taped on the wall and pieces of fallen tape on the floor needs to be fixed and cleaned up. The point is, if you are running a gym that its main objective is to attract customers and make money, it needs to take in account even details such as this, just like all the other types of businesses out there.
The list can go on, but I'll stop here. While I absolutely respect some gym's attention to detail and its headsetters in initializing a guideline to keep the bar high in routesetting standards, there are other gyms where I wonder if they are taking their business seriously. Basics such as keeping your main product in high quality and top condition is essential, and they are wondering if they need new carpet to attract more customers...
-Steve