I can certainly see your point from both an economic and a convenience way of looking at this.
On the other hand, I think there is a certain "thrill" some collector's receive from actually getting the books signed themselves and meeting their "hero" in person.
I don't personally like signed books, however, I can understand how other are attracted to them. I have had other things signed (movie -script and things like that, which do not have artwork)
If I were to get a book signed, I would want it to be on the inside.
Everyone has different tastes and each collection shows the personality of the collector.
So for each person, having a book signed is an individual choice. To sign or not to sign. To have it signed in person (as I would prefer and have done with things like a movie -script) or to save the hassle and buy it already signed (and hope the signature is legit). To sign on the cover or inside. To sign a high grade book or a lower grade. To sign a high dollar key or something less desirable.
I am thankful that people are different and so are their choices. This makes collecting fun and interesting.