Why do people say low power amps should be good for bookshelf speakers?

I was in a BB Magnolia recently and they had a McIntosh MHA150 integrated headphone amp that can also do 50 watts per channel to speakers. The sales rep said it "should be good for small bookshelves but its not enough power for towers". 
I've never understood this line of reasoning.  Towers are typically more sensitive than bookshelves. Is there an actual reason a small amp like this couldn't do just fine for towers that are equally or more sensitive than similar bookshelves?