If you do any exercise incorrectly it will lead to injury. I don't know why you think the wrist curl is inefficient, (although there is debate about whether forearms need a separate exercise due to the fact they are worked when doing bicep curls). All exercise produces results and as far as I am aware there isn't a precise way of measuring the efficiency. I don’t know if it will improve/help piano playing though, but it will increase the strength of your grip!
Yes, and the older approaches we grew up with do work, but there are much better methods now, that have more awareness of individual differences (and unlike the magazine articles, that will work even if you do not use steroids.)
Incorrectly could mean bad form of course, but more likely a bad balance of volume, intensity, and frequency. Those are the factors you must balance to get to progress.
By efficiency I mean finding the minimum amount of work to make gains. Body builders enjoy spending four hours a day in the gym. 3.5 of that is unnecessary, but they train to be able to handle it, and besides it's fun. Rather than training to build up your capacity, I want to train smart and make progress with the least effort.
To do that, you focus your routine on infrequent intense exercise, using compound multijoint movements (like squat, deadlift, clean, etc.) You never do isolation exercises like wrist curls or bicep curls, and you never never do high repetition light weight exercises. These are too inefficient and too prone to produce CTD injuries. They do not stress your system so they don't take advantage of some of the synergistic benefits. You don't do 12 bodyparts, with two to eight sets each. You do 2 to 4 compound joint movements, 1 set. 2 sets max. If you are really working out properly, you aren't capable of doing more than this.
Now I'll define intensity. Unlike efficiency you can do this mathematically. It is (percent inroad)/(time).
Volume is how many reps and sets you do. Strange as it may seem, you are far more likely to injure yourself lifting a light weight many times and getting fatigued and breaking form, than doing very heavy weights, even singles. (Singles are weights you can lift only one time.)
Frequency. If you lift weights on Monday and can't lift more weight on Wednesday, then one of two things are wrong. You wimped out and didn't lift hard enough Monday, OR (and far more likely) you didn't rest long enough. You should be able to make your workouts intense enough, and short enough, that 20 minutes once a week is enough to continue strength gains for a long time.
Really, to do this right, you need to approach it scientifically, keeping a careful log and finding out how to keep making progress. The typical YMCA lifter does the same weight year after year and convinces himself he's doing something good. Nah, he's just wasting time.
Research HIT a bit. There's some good stuff now. There's also some controversy, and there are people who don't need it. But there's no doubt it's the most bang for the least amount of time.
Topic: Gripmaster for strong fingers?