As for quantifying how much "home-field" helps your team, I'm sure WifS can figure it out based on simming full seasons of worlds, and compare what the winning% was with no ratings boost, and what the % is with a 1%, 1.5%, 2%, etc boost.
Also, there are many things in this game that take place that we quantify, that we can't in real life. What are the chances you get hurt with a low fatigue? Obviously, a better chance when your fatigue is at 90% than 100%, but how much? How much worse is a 65 stamina pitcher after 100 pitches thrown rather than 0 pitches? We don't know what the actual number should be, but we can make estimates.
The bigger question which you all bring up is whether this is a good thing for the game in general. Personally, I like to make this game as close to real life as we can. Obviously, a line has to be drawn somewhere, but I'm arguing the line should be moved. No, we don't have an artificial boost/fall based on hangovers and road trips that are longer than others. But we do have ratings changes based on events that occur besides the obvious - when players get demoted, when managers get tossed (I think?), when players don't get playing time...why do these changes take place and a temporary ratings change can't take place when you're playing home games?
This game does an amazing job of replicating real life. Home-field advantage is, IMO, the one major difference between HBD and MLB (besides one actually being real). I feel its a significant missing part.