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

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. 

Hi Robert -

It is a combination of factors ... right?  

1) Speaker sensitivity for sure.  2) The amplifier's continuous power output per channel measured across the full frequency range (WPC, 20 - 20,000 hz).  3) The size of your listening space.  4) How loud you like to listen or better said, are allowed to listen (apartment house vs private house, family complaints).  5) Minimum vs Maximum Power.  You should have more amplifier power than you really need as you don't want to max out or floor your amp.  Need clean power.   

It is not a tower vs bookshelf thing, per se.  It's how all these factors come together.  I assume that this is for music and not home theater, as HT really eats up power.  

Sales rep did not know his stuff or is compensated to direct sales in a certain way.

Best, Rich