Well Johnson is hitting 6th-7th in the lineup these days so he's not expected to do much and he isn't making a ton of money.
Most of the criticism with Uggla is due to the financial component. LaStella is a cheaper option, and in theory he'd hit leadoff which would slide Jason to 2nd, BJ to 6th or 7th, and so forth.
Uggla's defense is what is bugging me personally. Johnson at least doesn't appear to try to be casual with everything. Uggla's casualness, his lack of arm strength (or maybe he's holding back IDK), and his range are what kills me. If he hit as many homers as he did last year to start off you'd have no complaints from me about his defense. But if he isn't going to be producing on offense, the least he can do is not screw up on defense. Johnson may not be a great defender, but he's no butcher like Uggs or the Uggs as of late. Uggla isn't walking much and isn't homering, two of the main reasons we brought him here. Now that those two factors are becoming nonexistent with Dan, don't screw up on defense.