I agree and disagree. You're correct that all necessary information is contained in a single file. However, if I were migrating (as the OP is), I'd want to be able to say "Hey, I found the following Bug_Comments.csv record that does not have an associated Bugs.csv record!".
There may be more information in the first file that will be merged, but was not not being provided for the sake of brevity. In any event, I'm being pedantic. 8-)