I don't think it's so much a question of whether serger thread is thinner than regular thread. I think that serger thread is made up of short-staple fibers, which means that it will break more easily and shed more fuzz.
When serging, the weak thread isn't such a problem because there are 3-, 4- or 5-threads working together to hold the seam.
On a sewing machine, there are only 2 threads working together to secure the seam. That may or may not be a problem with serger thread, depending on how much stress is put on that seam. I think on most regular cotton quilts it would be fine for piecing; once a quilt is quilted, there isn't much stress on any one individual piecing seam. However, for a denim quilt that won't be quilted, each pieced seam is going to be under a lot of stress -- both because there will be no quilting, and also because denim is so heavy. On a small quilt, again this might not be a problem because the weight of the quilt even when wet might not be too much. If it's a large denim quilt, though, and there is no quilting, just picking it up will put a lot of stress on piecing seams from the cumulative weight of the quilt. If the denim is wet, the stress weight on those piecing seams will be even greater.
I think using serger thread on a denim quilt works if you are actually serging the seams together. I think the risk comes in when you use serger thread in a sewing machine to piece denim. Look at the weight of the thread used in denim jeans to secure hems; it's pretty thick! Also, all the stressed seams are double-sewn. The reason for the thick thread and double-sewing is so those seams will take lots of abuse and not come apart. A large denim quilt that is dragged around is going to need to have seams that can take a lot of abuse too, especially because of the weight of the fabric.
Just my musings here......