This is a good question. I think there is now the option to have multiple servers all running cron. Those might be your web servers, or might be some other servers.
It still seems plausible to me that really big jobs (E.g. forum emails, enrolment processing) are better on a separate server where you can configure PHP settings (memory limit, time-out, ...) differenty, but that is just a guess.
I don't know how much scope there is in the scheduled tasks system to control which tasks run on which servers. It might be possible to add that if it is not there already.