Practical to use Occlusion Query as sync object?

Hi all,

Some questions related to synchronization:

ARB_Sync looks really good, I see that you can now check for completion without blocking (for very long) or flushing - but is ARB_Sync likely to be widely backwards supported?

In the absence of ARBSync, or some kine of explicit fence, are there any major reasons why one couldn’t just use an occlusion query to tell when a particular piece of work had finished?