In old days every farmer in our village had a beehive - the honey and pollination services were both much appreciated. My father has always taught me that killing a bee is a horrible crime.
Can you support those claims?
Which is to say: they're so important that local populations of pollinators are inadequate to requirements, so they have to be supplemented by these mobile colonies.
Oh... Another reference: http://www.pnas.org/content/99/26/16812.full - you can check references [6,7] yourself.
Edit: I apologize - since I did limit my rant to honeybees initially.
The 2nd reference is the best so far, it provides some quantitative information about how dominant the honey bee is as a pollinator of commercial crops.
Personally I think bees are hugely important and it's quite sad the way bee populations have been mistreated, but it doesn't help to overstate the case (the case for being kind to bees is already very strong).
Bees are actively used for guided pollination, which is not possible with most other insects (not withstanding some experiments with bumblebees in greenhouses). Interesting fact is that you will often find the phrase "crop xxx requires yyy hives" while describing the role of bees in pollination.
hth.
