The benefits of using a series of podcasts to cover a particular subject are twofold:

1.  Delivery
One thing you do not want to do is stretch your 15 minute podcast to an hour and a half to cover your subject well.  If your listeners subscribe to a typically 15 minute podcast, then they expect to hear a podcast that lasts between 10 and 25 minutes.  If you go much longer they’lltuneout.  Of course some podcasts are longer, but that works because that’s what their listeners have come to expect. 

And, your listeners can’t absorb that much either.  Some topics need to be broken down for absorbtion purposes.

2.  Promotion
Consider that a series of podcasts are good for archiving into one complete ‘album’ that would be free on iTunes.  And, what better way to bring someone back for more than to make them feel they can’t miss a segment of a complete set? 

Series are also great for getting advertisers.  If you have a moderately popular financial podcast and for the next 2 months you are covering Dollar Cost Averaging in depth, you have a unique opportunity to offer an advertiser a 2 month trial advertising on that series.  Much like traditional programming, you can say that this Dollar Cost Averaging series was brought to you by ________________.    And, because series tend to be quite popular, you’ll stand a better chance of picking up the advertiser.