Ive had the same problem, here's my thinking on this now: basically, anything that raises yor internal temp by 2 degrees is a warmup...so kind of like squatting heavy will affect your overall muscularity, warming up your entire body will cause your (insert individual body part here) to be ready to go!
Now dont get me wrong, I'm not above doing a few lighter sets to get my head around what I'm doing, but I dont think warming up a body part by performing multiple (lighter) sets of your heavy compound exercise for that bodypart is the most effective method.
For instance, on leg day i might sit in the hot tub for 10 minutes or so then hit the bag OR do 4-5 sets of ab exercises...in between sets of these I'm getting the bar ready for my squats...then I'll do something like this
1x135x8
1x225x8
Thats it. I'm good and warm...maybe even got a little sweat going...shoulders, wrists, back and legs got plenty of blood flow and I've found my groove mentally. From here, I'll bang out 3-4 more sets of squats and move on.
I just have to stay away from the bikes or a bunch of lighter sets of squats or my strength suffers.