You can't look at things like that.
You can say that the improvement is giving 2 science bonus and trading 3 other yields for 3 science. Then I'd guess you would be right (although it could be 3/2 on lower yield tiles) but if you see it this way then you'd find that the bonus just isn't that enticing... 2 free science and trading 3 others for 3 science? for a great scientist?
The question is what else you can do with a Great Scientist. If all of the benefits of Great People are pretty marginal, then this might be the best of them, even if it's not that big a deal.
Not sure how marginalizing Great People will play, though. They seem to be one of the principle benefits of a small civilization, so if they're too weak, that strategy won't be viable. Granted it might not be anyway, but it'd be cool if there were at least some maps/situations where staying at 3ish cities and heavily developing them was your best option.