Lactic acid bacteria (LAB) such as
lactobacillus are seen to be beneficial to plants. They help combat diseases and pathogenic soil bacteria in much the same way that they do in our digestive tracts. I believe kefir usually contains a large proportion of these bacteria so I don't see why it wouldn't be a good thing to add to your soil.
According to
this study, LAB can also accelerate the breakdown of material in soils which leads to greater (composted, as opposed to complex and unavailable) organic matter.