Right now, we use Pivotal and the lack of estimation on tasks makes it an incomplete offering. We estimate feature, but that is a fairly rough estimate used for longer-term prediction. It isn't until we break things down into tasks that people really understand how long it is going to take. Just having to estimate hours gets people to really think things through.
Given that half of project management is about predicting completion dates and judging risk of slippage, I'd say estimation is absolutely required.