Why ? Let's face facts here : humans can't do a lot of things. There are so many useful things that AIs could do, from space exploration to cheap food and housing, to deep sea operations that humans can never hope to do.
General AI will be a massive advance for our economy, for our culture, for science, for military, for ...
A lot of things humans want to do but can't, effectively because of human body and/or brain limitations. From efficiently building buildings, taking risks that our bodies effectively don't allow for (e.g. being abandoned on Mars with little equipment would be a bit harsh, but not catastrophic, for an AI. And bringing him back means a data transmission), doing things that our bodies don't allow for (like building buildings/houses/... using huge premade blocks quickly. Humans can do it, but if we had hands the size of cars we could build those houses like we build lego houses). Defense/policing. An AI would not be risking life nor limb. An AI could just walk into the middle of a firefight, and worst case, he/she needs to be restored from backup)
All of these things sound like very good things. And yes, in the very long term AIs will replace humans. But in the very long term the human species is dead anyway. Does it really matter that much if we get replaced by a subspecies (best case scenario), another species, or AI ? Plus, you won't experience that, nor will your great-great-great-great grandchildren. At some point it doesn't matter anymore.