Along with the many benefits of evolution, have come many advanced concepts only as we as humans could conceive. The one in question being "Morality".
I don't know how we came up with it, but the idea is basically to limit one's actions, even without fear of consequence. For most, this system is built into our daily lives and limits us from doing things we consider to be in bad judgement, sometimes to be "wrong" or so far as to say "evil.
Morality does limit us in good ways and bad however. While it stops people from agreeing with your anti-handicapped plans, it would also stop people from beating their wives or children. Breaking down our morals would cause us to become more logical, yes, but also more "cold" inside and less humane.
While you do make a good point that the mentally handicapped serve to purpose but a hindrance, many a time it can't be diagnosed early enough for an abortion and people's general morality attempts to stop you from catagorizing them as less human or incapable, even if they infact are.
Say we were, hypothetically, to abort or isolate all mentally handicapped people. Next we'd have to do the same for physically handicapped people as they too can not function at full potential. Next we may need to isolate those who can not pass our standardized testing, as they to would have to be considered mentally inferior. You can see where this is going.
As an end product, you will have what can be compared to a well built machine. Each peice doing their job accordingly to maximize production and minimize waste. While this may be true, the overall lives of your citizens will change, causing some to look down on others, causing some to fear persecution of being "inadequate". It would be a system more or less run by fear, as any sense of compation and "morality" would be gone.
To, if you haven't figured out why people are apposing your idea so much, now you know. While it's unknown if, by your ideals being put in place, the country would be more productive or not, the majority of people won't even let you try.