Why didn't they? Couldn't a team of Spartan-II's escorting some naval personnel board a Covie Battlecruiser and capture it then allow the naval personnel to take control of it? Rinse and repeat.
I mean we all know that UNSC ships are garbage when facing Covenant ships. So they should've just captured Covie ships to level the playing field.
Would've been pretty effective in my opinion.
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They were hard to reverse engineer. The Needler is impossible to reverse engineer according to Halsey, and they don't know how to charge plasma weapons. Covenant ships are also near impossible to capture. [Edited on 10.09.2012 7:15 AM PDT]