Well, essentially shields are forcefields by any other name and they've been used in a variety of different applications in various Trek episodes from defensive, to containment, to structural integrity. Given that no order was given to drop the shields around the barge, it seems that they remained around it during the flight. Which is really why the use of the shields to help keep the otherwise disintegrating barge together is the most plausible idea. They certainly weren't for protecting the Enterprise from the radiation (wouldn't it have been better not to stretch them around the barge in that case?) and if they weren't considered necessary for going through the asteroid field in the original plan. I believe in the above transcript, LaForge was simply stating their current situation while thinking what options were available to him in regards to the tractor beam. But it can also be looked at as being a case that they had one system (a tractor beam) operating within another (the deflector shields) and that latter was having an impact on the former. If that was truly the case, it really wouldn't have been necessary to get rid of the barge. They could have merely positioned it at any point clear of the planet and left it there indefinitely. But--and I believe this to be most likely--it was a case that they wanted an environmentally-free system (no irradiated zones or spacelanes), then getting completely rid of the barge was the thing to do. It just turned out to be more troublesome than they thought it would be and dangerous than they liked it to be.