From a production point of view, where is the advantage ? I would have thought that genetic variety was safer and less disease prone in the long term.

The problem is that the more genetically homogeneous a population is, the greater the susceptibility to any pathogen that gets into it - the standard risk of monocultures. Also, a production-enhancing change in a genome can come at the expense of some feature that promotes hardiness. The production advantage is that producers hope the new, less hardy creatures will produce either more product, more valuable product (e.g. leaner meat) or the same product in a shorter time.

The fact is that what we're experiencing right now is a top-down disaster. -Paul Krugman