The first person who writes a really, REALLY long, detailed, thoughtful response to the OP is going to break the thread. 
Am I going to try to be that person? Well...
It's an interesting point, OP, and I think several people have advanced some valid arguments both for and against the practice. I won't reiterate them here.
To properly address the issue, we first need to examine the etymology of the word 'this'. Wikipedia tells us that the word originates "[f]rom Middle English this, from Old English þis (neuter demonstrative), from North Sea Germanic base þa- "that", from Proto-Germanic þat, from Proto-Indo-European tód, extended form of demonstrative base to-; + North Sea Germanic definitive suffix -s, from Proto-Indo-European *só (“this, that”)."
So there's that.
Next, let's explore the origins and purpose of the website 'MumsNet'. Again, Wikipedia is our friend here.
Mumsnet was created by Justine Roberts who came up with the idea of a website to help parents pool information and advice following a disastrous first family holiday with her one-year-old twins. Once back in the UK, Roberts persuaded friends Carrie Longton and Steven Cassidy to help her build the site that is now regarded as one of the most influential women's sites in the UK.
In November 2009, the Prime Minister Gordon Brown, opposition leader David Cameron and many leading ministers took part in live webchats with Mumsnet users.
Mumsnet's 10th birthday party was hosted by Google UK at their London headquarters in March 2010. Guests included Ed Miliband and Steve Hilton, and both the then Prime Minister, Gordon Brown, and his wife Sarah Brown gave speeches. Gordon Brown referred to Mumsnet as one of the great British institutions. In May 2011 Roberts founded Gransnet, a sister site to Mumsnet for the over-50s.
Roberts, CEO, was named in the Media Guardian's 2010 power 100. In February 2013 Roberts and co-founder, Carrie Longton, were assessed as the 7th most powerful women in the United Kingdom by Woman's Hour on BBC Radio 4. Roberts was appointed Commander of the Order of the British Empire (CBE) in the 2017 New Year Honours for services to the economy.
In 2018 Mumsnet had 1.3 billion page views from 119 million unique users, and revenue of £8.6 million.
In April 2020, Mumsnet announced a premium option.
Also of interest is the 'favourite biscuits' section:
David Cameron Oatcakes (Harrods brand)
Gordon Brown Did not answer (
)
Nick Clegg Rich tea and Hob Nobs (greedy)
Ed Miliband Jaffa Cakes
Boris Johnson Chocolate Digestive (prick)
Nicola Sturgeon Tunnock's Caramel wafer
Nigel Farage Did not answer (prick)
Jeremy Corbyn Shortbread
John McDonnell Broken rich tea biscuits from his Mum's counter in BHS
Anna Soubry Ginger Nuts
But I think, ultimately, we need to consider the Talk Guidelines, which state:
No personal attacks
No posts that break the law, including hate speech of any kind
No trolling, misleading or deliberately inflammatory behaviour
No trollhunting
No spamming
Now, I think the practice you describe could not reasonably be described as a personal attack, hate speech, trolling or trollhunting.
'Spamming' is an interesting one. It is defined as 'sending the same message indiscriminately to (a large number of internet users)'. I think the key word here is 'indiscriminately'. If one were to quote a particular post, followed by the word 'This' on a large number of threads across MumsNet, then that would certainly be in breach of the Guidelines. However, I do not believe that quoting the same post on a single thread constitutes spamming, as the post would remain relevant and repeating it is not, therefore, indiscriminate.
But.... does the aforementioned practice (i.e. quoting a pp and then typing 'This') constitute deliberately inflammatory behaviour? Well, not ordinarily. However, IF someone were to start a thread professing their intense dislike of the aforementioned practice, and a number of posters chose to display the very same behaviour on the thread in question (perhaps by quoting a lengthy and detailed response in exactly the manner the OP finds most objectionable) then I think an argument could be made that the aforementioned practice does, in fact, breach the Talk Guidelines.
I do hope this helps.