As far as starting goes, the system doesn't really count the nickel back fairly. Chapelle played 804 plays last year to Smith's 801 & Brown's 699. He played 824 plays in '08 and totaled 2,509 over 46 games played. If Smith & Brown play 824 next year, they won't match that, both in the 2,000 - 2,100 range.
I agree, all 3 of these guys have made an impact. On a performance basis, his numbers stack up as well. He made a 3rd down stop every 67 plays to Smith's 101 & Brown's 91 plays. His pass break ups per game of 0.5 aren't far off of Smith's 0.6 and Brown's 0.7. But you are right about matching up against the taller recievers. Chapelle had a Pass breakup every 100 plays, Smith every 82 and Brown impressively every 62 plays. He had 4 Ints while causing 2. Smith and Jalil have 3 Ints a piece over 3 years. So in line with Chap here. Again in TD saves he lags rhe taller guys with 2 vs Smith's 4 and Jalil's 10! On a Total Tackles basis Chapelle had 257 vs Smith's 113 and Jalil's 121. I doubt either of them will top 100 next, nor should they as traditional CBs.
But maybe it's not fair to compare the nickel spot to the others. Totaling Sacks, Tackles for Loss, and for Zero Yards Chapelle has 39 of these to Jimmy & Jalil's 8. This along with his overall numbers over 46 games gives Chapelle the nod for me. Though other two are great as well imo. Jalil might even have more upside than Jimmy Smith. I think 6-7 of his 19 pass break ups in last year alone were dropped Ints. Hopefully he'll mange to hold on to them next year.