The Empire would not be strengthened by allowing Aliens to consider themselves equal to humans... or god forbid purebloods. The empire will not be strengthened the day that it shares power with the alien barbarians. Give them an inch and they'll take a mile. The Sith and it's Empire is not there to be 'liked', and Imperial society is certainly not suppose to be equal..... Well that's the Sith's argument anyway.
While the Empire may be physically strengthened by allowing more aliens into their ranks, it would fundamentally change the Empire - the Sith and Imperial aristocracy would fade away. That would have weakened the xenophobic and tradition based Empire that said Sith/Imperials professed to strengthen, and create a new empire which is sickening to those currently in power.
You are probably correct. Heck, I have heard some people saying that the republic from the prequel movies is in fact the empire from this timeline, after having undergone subtle changes over 3000 years that, over time, caused it to become the very thing they fought against. (Said people typicly point at simalartys between the imp flag and the prequel flag, as well as other things