I guess I'll give my two cents. As a newer player (played on and off last year and have really started to work myself this year), I've found field practice so incredibly useful to improving my game. I guess it's just like ball golf (which I've played for 16 years) in that you don't run out to the course the day you buy your first set of clubs and start hacking away at the ball. You really need to spend time at the driving range to get a feel for it all. I don't think disc golf is any different (well, a little...grab your first set of 3 discs and go play a round to get pumped/hooked on the sport, haha) - I feel that [as a newer player], you need to spend time throwing discs in the field to get the fundamentals down.
When I go to a field, I pick a tree in the distance as my target - this way, I AM throwing with a purpose outside of distance. I work to improve my form, power, and my accuracy/consistency. To go out to a field and chuck discs around with ONLY power in mind is not helping you unless you're already spot on with your accuracy.
The other HUGE thing going to a field has done for me: teach me to relax. When you step up to a tee box, you see the basket and pull out your long distance driver, pick your line, and run up and power that disc into oblivion. For better players, this is great - I play in a league every Thursday, and the experienced players throw a nearly straight shot 400'+, layup a solid approach, and putt in for par. For me, when I was trying to muscle my shots (just like in ball golf), I was not only shanking my throws, but I wasn't getting near the distance I was in the field with a simple step and pull. I realized in my practice that if I just take it easy and work my natural rhythm, I was going to improve a LOT faster. After my last practice session, I literally knocked 14 strokes off of my average score at the local course later that day - it was amazing!
I guess field practice has a time and a place for everyone. Personally, I love going out to throw in a field. I've had nothing but success with my hours spent doing so. I don't think there's any better way to improve your foundation skills for the game. But, a field won't teach you how to throw that hyzer shot into OB, around a tree, back in bounds, right next to the basket for par. Things like that are learned through experience.