Well considering nothing is processed in Lab, considering the processing is done in ProPhoto RGB with a linear TRC gamma, just what good would having Lab values be?
If anything, having specific color space read outs WOULD be more useful than Lab because when you render the data, you end up in said color space. I’d rather have values based on what I’ll end up with. And since that’s not going to be Lab but will be ProPhoto RGB (or sRGB etc), seems more useful to have a particular RGB read out if desired (which we have in ACR).