So far I see the same issue myself. It is a bit comical and sad that no one from Microsoft seems to be addressing it. You can look through their articles and no solution in sight. This is what i have seen that may help.
If you enable the SQL Server Service for Kerberos then you seem to have a much easier time using a domain account for the agent, no local admin privledges needed:
Also if you specify the accounts during the install it seems to work fine as far as i can tell. It did on my Windows 2008 cluster with SQL Server 2008 R2 SP1. So it must do some special magic, i suspect dcom permissions and such, to get it working as i don't even see the recommended windows permissions afterwards on the SQL Server Agent account, yet it stops and starts just fine. I also enabled my clustered SQL for kerberos as this is also now a recommended best practice from Microsoft.
I was going nuts trying to get SQL Server to read a share off of another server for bulk insert until i read these article and got it working.
Hope this helps someone. And no i still don't have an answer for just directly assigning the needed permissions to get SQL Server agent running under a domain account independently of these other things without making it local admin.