We're getting into a pretty good discussion on this in the Advanced forum..
The gist of what I know is that plants become low on their own carbohydrate reserves late in flowering.. Its actually known that extended dark periods stimulate the plant to dump alot of carbs/sugars into the rhizosphere around the roots.. These carbs are necessary as a chemical energy source to perform chemical tasks to prepare raw nutrients to build amino-acids and other good things the plant needs to fully mature in a healthful manner.. Its pretty well known that carbohydrates can't travel through the roots, and up the xylem, but there are beneficial bacteria and protozoa etc in the rhizosphere/soil that feast on the energy they get from the carbohydrates, they can penetrate the intercellular region of the roots, and they can carry out various enzymatic reactions to prepare what the plant would have difficulty doing itself at this stage in its life cycle..
It seems to be a symbiotic relationship, but I'm not sure why the plant dumps its carb load.. I'm thinking it may be to prevent a runaway reaction that would leave the plant completely depleted of some necessary building blocks..
And molasses is very complex indeed.. It contains a whole lot of more complex sugars/carbs ontop of sucrose.. Yeast love it, it has everything they need.. I'm an avid fermenter, so I have alot of experience with this.. There are even better things than molasses for helping these little guys thrive.. Brew shops etc carry specialized micronutrients and energizers probably very similar to expensive carb loading suppliments..
Sucrose alone only contains hydrogen, carbon and oxygen.. Microorganisms can only use it as a raw source of oxygen/energy.. It lacks the complexity needed for building necessary molecules/ions..