This was a thought I had last night and it sounded smart then so please help me punch holes in it for those that are more awake.
I've been using Threat for testing for some time and in the meantime I've been playing with other dmg meter type addons as well (Violation, Recount, Assessment, etc).
A common problem with Damage meters is combat log range and the desire to extend that range by receiving messages from other people. I was thinking about how Threat-1.0 handles the sending of messages and leaves the addons that make use of it to display/store the messages and got to wondering if this type of capability would work for something like Parser so that each dmg meter doesn't have to have a proprietary sync with itself function. For example, Violation, recount and assessment users could all 'sync' with each other for combat log range purposes and would only need their own sync if they were storing data for a consistent raid wide dmg meter (like sw and dm does but violation does not). This way you still have a choice about how to display and store the data but you are sure you are at least seeing all the messages available.
I could be wrong, but doing this would cause a metric-shit-ton of AddOn traffic. And I know that I really don't care about my combat log being in sync with someone else when I only use Parser for SCT.
Your right in thinking that if they were synced, than the addons wouldnt need to worry about it. However its not the optimal solution, best to let the addons that actually need to sync worry about it themselves.
Edit: You can also simply up the range of your own combat log by setting a CVar, then your client won't miss those events.
Providing an overall sync lib might be nice, but it shouldn't be a part of ParserLib; it could have ParserLib as dependency but ParserLib in and of itself shouldn't have it included since there's a lot of AddOns which can make use of ParserLib that simply would never require synching.
So we could make, say, a mod called SwimmerLib (Swimmer because the first thing that popped in my mind at Synchronizing was Synchronized Swimming; don't ask), which is a generic handler for synching ParserLib related messages, which could than be added by other AddOns to handle the synching.
On the other hand, how many different mods do you usually run at a time that require the same kindof synching? I think this is mostly one of those things better left up to the discretion of individual mod develop[er/ment team]s.
Combat log range extension works most of the time, but there are a few encounters that are beyond the max range I believe.
Another library to fill this function perhaps? I know Violation has tools to receive SW and DM data so perhaps a library that does this that Assessment and Recount and others could use to handle synching?