Anything that prevents "unread" messages from coming over and over again makes me a super happy user!  
I'm glad to hear that  
I think that this feature could be good option for ppl who use facebook chat on few devices...
For example : i have messenger on my phone and tablet.  when i use miranda to talk by fb chat - it grab messages and these messages dont append on the mobile devices - its good ! but same issue, when i dont answer to user ----> chat is marked as unread and on site or mobile application i need to open conversation and check that i read it 
I dont know how much time this feature will  take from your life ( 

 ) but if this option is something like few lines code , please add it ^^
it will be really helpful