I didn't say it's harmless, I said you are highly unlikely to be able to measure an impact.

Now, what if you have something like Sharepoint that is known to deadlock (and it's not a problem), that traceflag was enabled by default leading to many, many MB of error log, potentially filling the drive?

Optimise for ad-hoc is not enabled by default for the same reason that any new option is not enabled by default, minimal surprises when upgrading versions. In scenarios where there's minimal ad-hoc query usage, or when I know that ad-hoc queries will always have plans able to be reused, I wouldn't enable it.