You could ask http://www.silverstripeclub.com/ how they are getting on...

From my perspective - I'd buy them if they matched (or would be customized to) my exact requirements. and maybe with the focus here on BSD license (apart from the help... booo...) makes it a different case, or maybe becasue silverstripe is a relative new-comer?

The BSD license is super flexible in this regard. As long as you don't claim your work has anything to do with SilverStripe Ltd, you can do as you wish.

Other modern CMSes, such as Concrete5 and Expression Engine have relatively healthy developer eco-systems for paid functionality, so I was wondering if it was a concious decision on the part of the SilverStripe community to not have something similar?

The idea of paid modules I believe is simply down to the business case. I personally used to be a Wordpress developer (mostly themes) and I could easily get ~100 downloads of a paid theme ($10) in a month without any marketing. With SS I'm not sure it has the market size for that sort of work to sell but it's getting there.

Would developers consider purchasing a module rather then using a free alternative? Assuming that the purchased version gave added benefits and was more professionally produced then free alternatives of course. I'm thinking bug fixes, support and upgrades etc.

It all depends on the quality of the modules, support and how well thought out the functionality is for whether it would be worth purchasing. Some module types such as ecommerce and events I feel would be suited to be paid modules due to the complexity, but recently releases have both been open sourced which is great.