we have embassies almost everywhere....the only exceptions I can name are cuba (mission only), iran (obvious), and DPRK (I think).
you lose nothing by having an embassy (unless marine guards let kgb agents in, etc), and by having one you can support espionage, diplomatic contacts, and in general keep talking to your adversary. Contrary to rhetoric here, you should always keep talking, even if you plan to bomb someone tomorrow..
I'm amused by the scrolling information board at the one in Cuba.
The thing with Syria, the United States is almost as much against them as Iran. Since there isn't an embassy in Iran (because of the hostages taken before Reagan can to power?), it would be somewhat understandable if the United States wouldn't have one in Syria.
As for your point of keeping up dialogue, that is of course very logical.