I don't think the Indian batting line-up is all that weak. We bat down to number 6 in terms of specialist batsmen (including Dhoni). Pathan is an all-rounder in the making. Harbhajan can throw his bat around for some last-minute runs, Kumble can apply himself if required. Piyush has been touted as a future all-rounder and Munaf even has a domestic 50. It's all different in international conditions, of course, but our tail can definitely contribute with the bat. That is what often rescued us against Sri Lanka, in fact.
Irfan Pathan, top score: 93, 50s: 5 (test matches)
Anil Kumble, top score: 88, 50s: 4 (test matches)
Harbhajan Singh, top score: 66, 50s: 2 (test matches)
Piyush Chawla, top score: 60, 50s: 3 (first class)
Munaf Patel, top score: 78, 50s: 1 (first class)
So, there is hope. Pathan averages close to 30, Harbhajan and Kumble add up to about 30 more. Piyush averages 25 in first class and Munaf averages 17. If we dare to take the risk of adding those up, they add up to around 100. So, going by solely averages, this bottom order is quite capable to add a few useful runs.
Compare this to England:
Harmison averages 11, Hoggard averages 8, Plunkett averages 21 (FC), Panesar averages 8 (FC). Only Plunkett has got a half-century, and that too in first class cricket. So the extra batsman could well be negated in that method, looking ONLY AT AVERAGES (probably not what will actually happen). However, playing a specialist batsman is always helpful since they can turn the tide of the match, something that our all-round tail is unlikely to do too often.