In the write up, you end up with a drone with armor 2 and body 1.... but if it was a shield, you'd add the shield armor to that... so armor 4 and body 1.
Hence why I said the badguy with a heavy pistol shoots the shield drone and destroys/disables it. 
I get it, but I don't feel its the right approach. Doing it like that indeed makes it useless. It doesnt have enough armor to even protect itself, let alone the rigger behind it. You'd be better off with a tacnet system and a load of small drones to gain you bonusses and distract the hell out of an enemy by opening fire from all sides.
So you would need to have either a much heavier wall in front of you that counts as actual cover instead of just a shield. Like a drone carrying a heavy security door kind of thing, which would provide actual cover and has a much higher armor and structure rating.
Or as Sichr says, you'd need energy shields. Which is interesting but I don't think anything like that exists in SR and I don't really see a science(fiction) based near future way to actually do that. A pretty powerful magnetic shield generator?... just imagine the power source needed and the generator itself. You'd get one massive drone. Only doable stationary if at all.
Or.. you'd have to go for a high tech defense system like a missile shield kind of thing, where attacks are (attempted) repelled by counter shots, lasers or whatever.
