I thought about it, but have some concerns about using a cooler as an HLT.
1) Since most sparge water is heated to 190+, there is a potential for warping the cooler. Also, if it takes awhile to get to the sparge, there might be some heat loss. Thing is, lots of people use them and they work, so what do I know.
I have a seperate burner and keggle for sparge water. The two pot/burner method works the best for me.