Consider following scenario:

script.sh in my repo:

echo $BAMBOO_BUILDNUMBER

Bamboo plan consisting of a single ScriptTask, which calls 'script.sh' from my repo. What happens? AFAIK the BAMBOO_BUILDNUMBER will be blank, where we would, tfu, Jenkins users could expect that BAMBOO_BUILDNUMBER will be some #no.
Note the $BAMBOO_BUILDNUMBER is environment variable, not the

${bamboo.buildNumber} 

This is contrary to our 'casual' plan/global variables (user created), which are injected into environment. If you create plan variable "plan.variable = 50", then calling script containing

echo $BAMBOO_PLAN_VARIABLE

will result in displaying 50. At least AFAIK.

Does it make sense?