Someone correct me if I am wrong, but honestly, take a step back and think about all of these LoRs applicants have. How often do you think a LoR has something
bad written about an applicant? How often does a prof get asked to write a LoR? I am pretty sure adcoms see the same adjectives thrown out over and over and over and over again in LoRs that they end up not bearing a whole lot of weight or play too big of a role in separating applicants from one another. Yes, I would personally love to get profs who know me inside and out to write my letters; it would mean a lot for me to know he or she has my back. In the end though, I just wouldn't stress it too much content-wise. It wouldn't hurt to share your personal statement/feelings about career choice among other things; if you can tie them into your goals and show that it would help you out a ton if they could write one for you, it will show, and I think that alone will get you above average positive things.

just IMHO