Look at a company like BASICmining, their share value reflected their current equipment without any kind of development of their own.  
You don't look at it and say "Oh this won't be profitable a year from now!".  Obviously every company is going to attempt to adapt, whether it be purchasing new equipment or making their own, but so far I have noticed that current hashrate is the dominating factor of share value.  

I should have added one last assumption:
"The company continues to adapt to increasing difficulty through various means".

Same with ASICminer.  You don't base your annual return projection on a constant hashrate for an entire year.  You expect them to continue growing, but the current hashrate is all you can base the true value on as adaptations aren't as predictable.  

In the end, the current dividends are what matters to people, and from there you can only make guesses on the miners ability to maintain this.  36% is incredible.