I think there's a lot of reasons I start with pre-draft. One reason I do is just so I have more time to work the rosters with regards to franchise tags, RFAs, etc as well as controlling my practice sessions.
Second, and most importantly, we already know who each team took in real life, doing it before gets things more varied, like having Matt Ryan on the Dolphins sometimes, seeing Cincinnati trade up and take McFadden, etc. I don't go through and take all the sleepers - and if I do take someone who ends up really highly rated, I'll take them early - I've taken Atyha Rubin in the 3rd round, for example. If I wanted Campbell, I'd take him in the top 10. I could make a whole draft out of 85+ rated players, but A: it's not realistic, and B: it's not fun. I do take busts if it "makes sense" and the GM wants it - like in this draft I'll post, I took Jason Jones (DE) and he's got like a 54 inj - but I needed a DE, he was the highest rated along with Quentin Groves (but I passed on Groves because he's too small for what I wanted story-wise), and a GM suggestion. It's easy to be fair if you try. Cap-wise, I could dump the same people after the draft - makes no difference. Some people might be different on the rosters between the two - but as far as $$$, it really doesn't matter. And if people are going to "cheat" the game, why play?
Lastly, I tend to get attached to guys and hang on to them, if I have a 80 rated corner who leads my team in interceptions 2 years in a row, I'm not suddenly cut him because there's a 90 rated guy in FA some team stupidly released and no one signed yet. Watch me when I get a future franchise started so I can go to win the Super Bowl with David Carr... (I'm a Fresno Fan. Heck, maybe I'll do it with Dilfer!)
Anyhow, hopefully it'll make more sense as I get rolling and I appreciate the questions... keep 'em coming! I'll try to get posted tonight.
You know maybe I'll try one post-draft to see - I haven't done one like that since the first months the game came out... not one that I even finished the first year of anyway before I got a new idea. We'll see how it goes.