Well, typically coins become more profitable when their difficulty drops.  It's usually not the result of market price fluctuation (although it can be, but the difficulty typically has a far larger effect).

Don't forget, submitted shares do not 'build' toward anything.  Each share is a random guess at the solution for the block.  Once a new block is found on the network of a particular coin, all the work done on the previous block is null and void.

So there is really no penalty for switching between coins.  If the multiport goes from LTC to WDC, all it really means is instead of submitting random guesses for LTC you are now submitting them for WDC.  A block should be found every X random guesses, on average.  (X depends on difficulty)