There are many things about the SATS that I don't like, but it isn't a bad thing for our kids to learn grammar.
If you ever intend to learn another language, then you do need to know basic grammar. I went to school in an era when we didn't learn any grammar at all. It has been a major hindrance as I have gone on to live and work in 3 different countries, and learnt the language in each one. It was an uphill struggle to learn basic grammar alongside learning a language.
Most of the things in the SPAG are good things for year 6 aged children to know:
subject verb object in a sentence
what is a noun, verb, adverb, adjective, preposition, connective etc
correct use of punctuation, including commas, colons, semi-colons, full stops etc
understanding question v. statement or command
direct speech v reported speech
active v. passive voice
route words of spelling, prefixes and suffixes
present/past tense
use of standard English
hyphens, brackets, dashes
correct use of capital letters
apostrophes
to me theses things are basic to good written English and should be taught. I have no problem with them.