Over the past couple of years I've been making solutions for SharePoint Server and always most important problem was the availability of the web-site. Deployment not-sandboxed solution is pain at least for 30 seconds for each wsp. This operation requires recycle of Application Pool (include the farm one) and in fact this means 503 Service Unavailable http response from IIS Server.
We usaully use SharePoint for external Internet web sites and we expect the highest possible percentage of availability. (Two farm for replication, powershell-automation, read-only mode while deploying, etc.)
Yesterday Microsoft released SharePoint 13 Preview. I've just downloaded slides1 and see there
Never any reason to have separate farms (Except security)
Any possibility for another deploy scheme?