I don't even care if it's 10 times in a row as long as the posts or topics are relevant and useful. Maybe not 10 in 5 minutes but that's what flood settings are for. 10 in a row, in 10 days, how/why would that be a problem? It doesn't prevent others from posting.
How did that even become a common rule?
I'm not sure to understand exactly what you mean, sorry for this. But I'll try to respond how I have understood starting from your words "10 times":
I don't like situations like the following in my board:
I absolutely prefer situations like the following:
[Of course in the images I used standard repetitive text, just for the example]
As well as everyone can see every situation represents 10 posts: the second situation where posts are merged is much more readable and pretty easy to handle. Nevertheless the second one is a better and more elegant and professional solution imho.
So imho if the option to activate the merge post feature is present into the ACP every admin can use or not as needed. If it is not present nobody can use even if needed...
Because the topic are related I gave the same response here too: