I really don't know why people use TV speakers. I think it's because 1) They don't allocate any budget towards audio and put it all on the TV, and 2) They are uneducated when it comes to audio, and are even (in my experience as a salesperson) intimidated by it. They think that decent sound has to be big, expensive, elaborate, or all of the above. A soundbar or HTIB is a step in the right direction, but I find that when people do want external sound when they're buying a TV, they view it as an all-or-nothing proposition - I usually direct people to forego buying that 5.1 Samsung 1,000,000,000 watt crapola home theater and start off with a receiver and a pair of bookshelf speakers, that way they can build their system up over time and end up with something much better in the long haul.
I recently added a cheaper Pioneer receiver and cheap pair of Pioneer bookshelf speakers to my secondary TV and think they're pretty great considering what I paid. It gives me real stereo separation, which a soundbar simply can't do as well, a real receiver with lots of inputs, and i'm able to crank it up if i want without feeling like it's being strained. And I can add speakers to it over time if I want.
ht Panasonic 60" ZT60, Monitor Audio: Silver RX6, RX Centre, Silver RX1; Martinlogan Dynamo 700, Marantz SR5006, PS3, Oppo BDP-103D
2ch Sony KDL-32W650A, Sonus faber Toy Monitor, REL T3, Marantz PM8004, Sony BDP-S1000ES, JVC T-X3 tuner, Apple TV, Peachtree Audio DAC•iT, Sennheiser HD598
pc Monitor Audio Radius 90HD, Audioengine D1, FiiO A1