This has answered a question that I was going to ask. As I have 5 pairs of padded undershorts I don't want to splash out on numerous pairs of bib tights.
I wear tights over padded shorts for that very reason; padded bib tights are more expensive than unpadded.
Plus that way, one or two pairs of tights are all that's needed. If you have padded tights and are washing them after each ride (as you perhaps ought), if you're commuting that means 5 maybe 6 pairs (unless you like doing regular washes) and then that does get seriously expensive. With unpadded tights, you can just put them over summer bib shorts/undershorts and wash all at the end of the week... or whenever there's a break in your work pattern.
To my way of thinking, it just makes more sense.