I'm a stickler for exercise form in my weight loss client's workouts. In fact, some of my clients think that I take exercise form too seriously.
But you can't fool around with exercise form. You must do each rep of each exercise with the best form you possible - even when you are tired and struggling to get through the set (that's where the real benefits are achieved!).
So you must go through the safe, full range of motion to get the most out of the exercise. By doing so, you will get more out of the exercise. And more work equals more results.
That's not hard to see, is it? I mean, if you go for full depth and perfect form on a squat or split squat or dumbbell press, we all know that is harder and takes more energy than to use poor form or to do only half the range of motion.
If you do an exercise with picture perfect form that means you've also done the exercise in its most difficult form. And that means you will gain the most muscle and lose the most fat when the exercise is done to its full capacity.
Poor form is often lazy form - its the easy way out - and this will not get you the results you need.
On the other hand, if your mobility (a better training term than flexibility) prevents you from doing the exercise, then you are best to work on your mobility with bodyweight exercises, such as the Spiderman Climb, Spiderman Pushup, Y-Squat, and the other metabolism boosting strength training bodyweight exercises.
By improving your mobility, you'll be able to do the Single-Leg Reaching Lunge with perfect form (i.e. lots of depth while maintaining a slight arch in your lower back).
I've added the Single-Leg Reaching Lunge to the bodyweight page...just scroll down to the exercises here.
Remember that all exercises should be done with your abs braced (as if someone was going to punch you in the stomach) and your low-back slightly arched, but never, ever rounded.
Train safely, train consistently, and train successfully for weight loss.