I do not think that will change. I think the roster, as of today, is essentially done. They will probably do their "bring in a few random relievers on MiLB deals" thing between now and camp, but I also think "what you see is what you get".
On the bench, I think they will be fine being that RHH heavy. Their lineup has a lot of left handed hitters that will likely sit against LHP - PCA, Busch, Ballesteros are all likely to see far less playing time against lefties. On those days, the Cubs will likely have a lot of left handed hitters on the bench. So I think it's a feature, not a bug.