Their purpose is to fill in ambience while one's attention is focussed on the screen and the front speakers. The bandwidth available to surrounds and heights is not very extended, nor does it need to be, and unless you are particularly keen on ignoring the main sound channels and concentrating on the emissions from the ambience channels alone then why worry?
This may have been true at the time of original Dolby Pro Logic but is less applicable in a Dolby Atmos context .I would say find a good dealer (there are a few that lurk here) and get a demo.
Overhead sound is a vital part of the Dolby Atmos experience... Dolby Atmos audio is mixed using discrete, full-range audio objects that may move around anywhere in three-dimensional space. With this in mind, overhead speakers should complement the frequency response, output, and power-handling capabilities of the listener-level speakers. Choose overhead speakers that are timbre matched as closely as possible to the primary listener-level speakers.
Even if it means biding your time, saving and doing research, better speakers will stand to you in the long term. If you aim too low for the wrong reasons you may not achieve the quality of sound you would enjoy most.
~M~