I run my new 9.2 channel Integra 50.4 AVR in the family room in a 5.1 configuration (front L&R, rear L&R, and center - no side channels) with subs and 2 additional height channels. My front L & R are Silverline Sonatinas and they sound quite good for music (on the rare occasion that this system is used for music).
However, the room came pre-wired with 2 speakers pretty high up in the walls next to the TV nook and 2 more near the back of the room in the ceiling. I assume they were intended as front L & R and rear channel speakers.
Since I use the Silverlines as front L & R, and I had the 2 built-in speakers that were already in the walls beside the TV nook, I decided to use that built-in "front" pair as height channels and the back pair as rear channels. They were already there so I figured I might as well use 'em. I didn't install built-in side speakers because they weren't already there and it didn't seem important to add 'em.
In my more elaborate HT, the 7.2 channel Integra is run in a straight Front L&R, Side L&R, Rear L&R, CC, dual sub 7.2 set-up. If the room is big enough, the extra 2 side channels are uselful. The height channels in a 9.2 can also be fun, but IMHO, neither is essential.
Marty