Zaheer wasn't that threatening before..in the tests. He was devastating a few times, and bowled exceptionally well against top teams, but failed to get too many wickets. Remember, India did not play too many test matches outside the sub-continent. He has a 5-for against NZ in NZ, one against Aus in Aus.
On Indian tracks, he bowled with commendable line/length (2004 Aussie series is a fine example) but just failed to get the wickets. He came back strongly in the 2006 Pakistan tests, he was the highest wicket taker and by far the best bowler (except Asif). But he failed in two ODI's where he was used more like a part-time bowler.
I didn't see logic in him being dropped. He has always performed on par with the expectations. and with him in a good form in county, his services will be needed. Especially when the team requires an experienced paceman to lead a new attack.