To use your example, if the price is 2.8 and you get charged a 0.5% trading fee. When you buy you are really paying 2.8140. And when you sell, you are getting that same 0.5% on the Bitcoin side. So if you sold at 2.8281, you are really selling at 2.8140. So you bought at 2.8 and sold at 2.8281 and managed to make a big 'ol goose egg for yourself.
You're missing the point. If you're providing this level of liquidity then you're very effectively cashing in on the majority of swings the market makes. The spacing of those bids and asks from the spacing you can see that this is being done by someone with one of the really low fee levels like 0.25% (someone correct me if I'm wrong, but I recall this as being the lowest). To do this effectively you need a fairly large amount of both btc and fiat but once you reach that level
Essentially the harder the market spikes in either direction the more this guy/gal is up. This is an extremely logical trading "system" provided that you A you have deepish pockets and B don't believe that bitcoin is going to kick the bucket anytime soon.
Providing liquidity like this can also earn very good profit if the fluctuation continues.
+1 Absolutely right