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?
roberthz
Even bookshelf speakers need power. 
  Run a old BIC d62  pair with a yammy receiver w a tiny 80 W RMS. 
      RAN THEM WITH MY SUNFIRE SIG 600, No I didn’t  kill them, I played them with their power as specs, sounded great, 

    Know the limit of the speakers, and how hard to push them.   Have 3 systems, main, 2nd and my 3rd which is BIC v-630’s with yammy 80 W receiver , hooked to my SL-1200MK2 to record to computer, with a few cartridges, Arkiv, and nightclub II TO RECORD LPS TO HARD DRIVE 
 USE audiolab 2.0.
  Then record to cd to play.

 I love the snaps and pops of the recordings!j


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I remember Platinum Audio bookshelf speakers. Real power hogs. Needed about 200 wpc to get them to sound right. 
Is speaker sensitivity the bottom line for determining which amp is sufficient? Or do the number of drivers in a tower become a factor? Do more drivers require a bigger amp for sufficient control?
Yeah don’t listen to Best Buy employees. 99.9% of the time the larger speakers are easier to drive as they have bigger drivers than need to move less for the same sound pressure.