For most I guess it would, but I would get frusterated if I wasn't reaching my goal "fast enough." Some would say set smaller goals then, but what if I reach them too soon then and don't practice as much as I could have if I just set a time limit? And if I'm going to set my goals so they fill up just the right amount of time, then I might as well just set a time instead, as that takes less thinking power!

It has a negative and a positive. The negative is, as soon as my time is up, I'm off whether I'm in a middle of a bar or not

. The positive is, if I finish up a page and still have 10 minutes left, I don't call it quits, but use those 10 minutes to get a head start on the next page, or to play over the learned pages instilling them further into my memory. So it evens out.
I have become more flexible in my "times," lately, meaning that I will say x amount of time for studies, x amount of time for sight reading and ear training, and x amount of time for pieces. I used to say x amount of time for each piece (changing the amount of time when necessary depending on the length, hardness, ect. of the piece in question) which worked really well because then the pieces that I didn't like still got practiced a good amount of time. I paid for my more lenient method in my last exam by blanking on a piece that I avoided practicing. This year I'm still staying more flexible by lumping my pieces into one amount of time again, but I'll make sure I don't neglect any this time!
I keep track of it in my head, but I used to do it on paper which was a good motivater because if I didn't complete my time one day, part of the paper would stay not colored in, and that bugged me so much that I would do "over time" the next day so I could go back and color it in.
It's because I was home schooled and my mom said I had to do 45 minutes a day for each subject. So I crammed as much of each subject into each set of 45 minutes as I could and when my time was up, I was out of school for the day. So I'm very used to working like this. I look at piano the same way. For me, anyway, it's not a hobby, it's my "job." (I'm a full time piano student at the moment). So if I was working I'd do x amount of hours per day, so I treat the piano the same. Within that time, if I'm tired of memorizing, I'll work on adding in dynamics to some already fluent pieces. Or if I want to be outside, I'll practice memorizing off the score and then going in and seeing what I retained. And there's always ear training, sight reading (though not too much because this can distract one from important practice), and technique to work on.
It can be dangerous because you can end up avoiding stuff, but I try to always keep my 6 pieces or whatever the number is at the same standard, meaning, yes it would be nice to play the already memorized movements of the sonata, but the 3rd movement isn't memorized yet and neither is the Liszt, or whatever. So do I feel like working on the 3rd movement or the Liszt? And if I get bored of these, I'll allow myself one run through of the memorized movements, ect. Once all is memorized, then I focus on the pieces that lack technically the most, then the ones that lack musically, ect. But I'm rambling. This is just what works for me. One last example. I happen to have 30 minutes of practice left tonight and I am dog tired. If I didn't set "times" I would easily let 30 minutes go. But I will tack it onto tomorrow's time as tomorrow is lesson day and has a slightly less amount of practice time due to this anyway. If it didn't, you can bet I'd be at that piano tonight regardless of how tired I was. If I was too tired to memorize, then I'd work on the other stuff. So it has benefits. You just have to be careful you don't abuse it and goof off during the time you're suppost to be practicing which I have been known to do.....
