You can already do this in RM. A typical example would be a trigger of "sensor X changes," then you'll split your actions sections using an IF ...ELSE ... END-IF. Your IF will test for one of the desired conditions (e.g., a contact sensor being open). You can achieve the "stays" part in different ways, but a typical example is to use a "Delay x minutes" action. You'll likely also want to set the "cancel" flag on this delay action and use a "Cancel delayed actions" action in your ELSE, which will un-schedule the commands set to run after the delay if the sensor state changes (truly making it a "stays," not a "was x minutes ago").
There are more examples in the docs, which I'd suggest reading.
There is nothing wrong with multiple rules. 
(They're free, as people often say--but you'll find this far less necessary in Rule 3.0 and later compared to earlier versions due to the actions being an ordered script and the introduction of conditionals.)
