His bat is not big enough (insert Wang joke here).
Unless he dramatically improves, his defense is not good enough to overcome the lack of offense he would bring. It's just not. LaRoche would come closer.
I could see Rizzo sending a bullpen arm to LA for him, though.
Take a look at Loney's splits even last year. .785 OPS and .340 wOBA against RHP. He didn't hit LHP at all. Go back through his first 3-4 years and you usually get those kinds of splits (LHP/RHP) but you get a bump away from LA. That's not nearly as bad a bat as the home park made it look.
Home/Away OPS and wOBA for Loney (06 is way too small a sample size):
07 -- .866/.939 OPS and .374/.397 wOBA
08 -- .735/.815 OPS and .320/.350 wOBA
09 -- .640/.859 OPS and .282/.375 wOBA
10 -- .717/.729 OPS and .311/.320 wOBA
I'm going to guess 2010 was the aberration and that Loney is a much better hitter away from LA and against RHP. He's a good platoon guy/defensive replacement to have around. There should be no problem running him out there for 400 PAs during a season.
That said, I'd obviously prefer LaRoche/Lee at 1B.