Keeping to a max of 10 broken ports at any time is nice.
However, there is one really significant difference between the FreeBSD ports tree and the OpenBSD one that should be noted: OpenBSD has about 2000 building packages out of a total of about 2000. FreeBSD has (as of the last INDEX I've got) about 10600 that builds of a total of about 11000. Sure, we've got 40 times more ports that do not build - but we've also got 5 times as many ports that DO build.
I don't really see the number of broken ports as a "To branch or not to branch" decision - it's more of a "port count" vs "maintenance quality" vs "resources available" issue.
The branch decision (FreeBSD not branching) is due to the perception of branching being too much work, and believing it to be less work to just maintain the single tree with support for all relevant branches. The perception of the OpenBSD team is obviously different (probably as a result of somewhat different goals.)
But big kudos to the OpenBSD team for the maintenance work - I think 0.5% breakage is extremely nice work.