Training & Nutrition - Is this bad policy?

Since daylight is now a bit longer (thankfully), I've been biking much more frequently, usually finishing up with a half scoop of Cytomax then hitting the dumbbells-nothing much though, just with 15,20,30 pounders. However, in the past few months, I would be lazy with the dumbbells and the day after workouts, I was never sore. Recently I decided that because I always exercise with music playing on my MP3, I would work a particular area with the weights for the duration of a song, and when the next song rolls around, I'd take that 3-4 minute break and shoot some hoops, walk around a bit, ect.. And boy the next day I was sore like I've never been at my biceps since, heck, freshman lacrosse tryouts in Jan of 05. I figure it works, though I don't know if there is any reason to be suspicious of this route. Usually I'll do three rounds per section, which makes about 9-12 minutes per section total.
Input, criticisms?

Good idea. You want to really work an area, do reps with a weight until failure, then drop the weight and do it again. Repeat. Don't do that very often, though.

Personally, I think it's a waste of time to use weights that are that light. If you're having fun, cool, but you're not getting any stronger.