I would argue that English is the best subject that it works in. The reason that this works is because of the standards, if your standards are too general or too hard to accomplish then it will not work. If a standard for Grammar Punctuation was "Places periods in appropriate spots after sentences" that would be a piece of cake for a normal 6th grader to do. If the standard was "Uses proper quotation and comma placement to structure a quote" that would be more leveled for 6th grade.
I can't speak for other schools but middle school teachers in our district use standards behind the scenes for their own purposes of tracking student knowledge and understanding. But they have to then take those standards and form a rubric that converts them into points to put in the grade book. Not only does that take more time but the simile I like to use with it is that you have to measure in inches and report it centimeters, kind of pointless.
Language Arts is subjective at times. For example, last year we studied the holocaust for a month. It wasn't social studies so the standards weren't to understand and grasp what happened in the holocaust but to understand the social effects, and dive through writing from the time period. In the grammar example I gave above you could grade them on a worksheet and give 5/5 points for secure,4/5 for developing. The issue with that is they got an 80% because they slipped on one problem and in the grade book that's the only thing that is reflective of that. Where when you use standards you take into account that worksheet, group work, independent practice, formative and summative assessments to decide what they receive on that one concept.
Everyone puts their own little spin on standards based reporting but it still comes back to the standard. In a standard of "understand and use the 201 concepts of grammar" everyone would be developing. Standards are sorted and specific and that's what makes them work.
I hope I didn't throw you under the bus too far Meg. :)