and of course, with anything, it depends upon the personality of the individual.
we all know people who will take the approach--i don't think that i can do it perfectly, so i'm not going to do it--in which case it is good to encourage trying and failing. for these types of folks, it's good to learn about how failure is a positive aspect, and so you actually have to teach them how to fail without shame.
for other personalities, 'trying' is an excuse to not put for right effort and thus neither succeed or fail. in a sense, they're avoiding success and failure. for these, the phrase functions well. you either put in enough effort to do it or you assert honestly that you're not going to do it, rather than whining (as is so often the case) "i'm trying!" when the right effort is obviously not being applied.
so, in this family, as with luke skywalker in training with yoda, this phrase and method may be functional. my husband, who is of the same personality type as luke skywalker (who is George Lucas, btw), reminds himself of this phrase when he is being too passive in an effort to avoid failure and success, when whining "i am trying" when he is not, and so on. so, it works for him.
it may not work for everyone, but there is wisdom in it's use.