I've seen claims that 9-BBN can react with internal alkenes, but I haven't attempted it myself. I imagine the reaction would take longer than a reaction with a terminal alkene. The goal of the big substituents is to favor boron addition at less sterically hindered sites with high regioselectivity. 9-BBN generally tends to be more regioselective than something like borane-THF. You can also get it in solid form, which is easier to use than gaseous B2H6.