Do you think there is a certain reason why Henry (client from HoB) was allowed to sit in Johns chair when he was presenting his case to Sherlock and John?
Sherlock isn’t fussy about John’s chair until John gets married. Lots of people sit in John’s chair until then–Mrs. Hudson, Mycroft, Irene. He even offers it to Moriarty (though Moriarty takes Sherlock’s chair in a power play). Sherlock doesn’t get territorial about it until after the wedding when he doesn’t want to be reminded of John, when he no longer wants to see it (and possibly doesn’t want Janine to sit in it…though that’s speculative).
Also with Henry Knight in John’s chair, from Henry’s POV we get the shot of Sherlock pumping his entire body craving “cigarettes” and then John gets an eyeful of Sherlock leaning over Henry (aka John if he were in his chair) hungrily sniffing…so much pent-up sexual bromance tension all round.