Comsumer pooling in JMS

I was going through how JMS can be configured using Springs. In one of my previous posts on consumer pooling in JMS I was told I can use any of these. Can anyone please highlight what are the pros and cons of each of these and when to use which.

Also how to go about consumer side pooling in case I don't wish to use springs. Another thing I was told that it is difficult to do consumer pooling because
"The main problem with consumer pooling is that if you don't consume
messages when the broker dispatches them to the consumer, they will
stay there until the consumer is closed. So you can not really have
consumers waiting in a pool."

My question is when can this scenario arise. Doesn't I have a Message listener registered always and isn't the listener suppsd to take care of handling each msg as and when it arrives.

Regards
Atin Sood