As far as I know, yes, it's for the sake of simplicity to avoid filling out the extra return, making a copy, using an envelope/stamp, etc.
I had a related situation one year where my three kids each had capital losses they needed to file for. Since I am cheap, I filled out, copied, and mailed eight paper tax returns, which was kind of a pain compared to the two I normally do. Also in my state they assess $10 for each tax return filed, so it cost me $30 extra to pay the $1 per kid income tax owed.