Jim and Blut Wump,
Those are great posts. Now that Blut Wump brings it up, it is important to know whether or not the hips are a weakness. If they are, then they need to be addressed. This kind of falls along in a discussion of why Westside has been ineffective for some people, and it is because they did accessory work to address weaknesses that weren't their own. Personally, in beginners, if their knees come together on a squat or their lockout is weak, I feel just doing the exercise with a weight that allows good form and increasing resistance from workout to workout is all that is needed. In more advanced athletes, I am a fan of addressing weak points with other movements.
Jim, as far as machines go to address weaknesses, it gets tricky. You have to really have a good, solid understanding of physiolgy to avoid wasting your time...For example, I think adductors/abductors are good to address weak hips by nature of how they strengthen the area and how those muscles act in a squat. You need to be able to pick up on a weak link, diagnose it , and know the treatments so to speak for it. A lot of people just figure, hell, leg presses help your squat because they hit the legs, but this really is not the case. They get the legs to grow, but have little carryover to squatting strength. If you squat PL-Style I think abdutors/adductors are very good , but, if you squat olympic style and the hips become weak, I'd just use PL-style squats as assistance to strengthen the area since your hips probably get little stimulus from your workouts anyway. If you are getting 'folded in' by the weight, I'd suggest weighted ab work and heavy good mornings. Things like that.
So, basically, you need to know how to determine if you have a weakness and what it is, and then understand if an exercise has practical, applicable carryover to the weak area or if it is just an exercise good for cosmetic purposes. Trial and error may even be involved. Also, a lot of times people misinterpret, and they will think "Wow!! abductors and adductors help your squat, I will do them and get a huge squat!!"....but if they have strong hips already they are wasting their time in my opinion. Once someone gets to an advanced level, it is a game of making a weakness a strength, then another new weak link crops up that needs to be addressed, and so on.
This can get controversial and heated too. Look at Bulgarian Olympic lifters, who do zero assistance work and just do the contested lifts, then look at how the Soviets trained, where variety was the spice of life. Both were successful.