There are already numerous frameworks available for RapidWeaver that let you build upon the wonderful Stacks plugin from YourHead software. Foundation and Foundry are the main options available though there are others (UIKit in particular is worthy of checking out). These frameworks let you build freeform web pages in RapidWeaver that quite simply aren't possible otherwise.
When I first came to RapidWeaver (a couple of years back) I opted for Foundry and I still think it is a great choice. Initially (as I think is true for most newcomers) I tried to work completely within the framework, using only the included stacks. What I soon found however was that, in order to build the sites and layouts that I wanted, I was needing to turn to third-party stacks. This was happening more and more and it reached a stage where I was only really using Foundry to set up my fonts and colour schemes etc. This, of course, seemed rather wasteful; loading a whole framework (in Foundry's case it's Bootstrap) to set up these basics. And so the idea for a 'micro' framework was born.
Source does not try to be all things to all people. All it does it provide a solid base that I can use to set up my fonts and colours and then simply get on with building my web pages using my favourite stacks (and/or the small bank of included Source stacks), safe in the knowledge that the framework isn't adding unnescessary overhead to my pages (which of course has an impact on speed in Edit and Preview modes as well as on the published site).
So, to summarise, here's why I feel there is a place for Source at the framework table:
Source is lightweight: Source is really only there to set up the basics for your sites. Mainly fonts and colours but it also sets up some default stylings for form elements, quotes, lists, tables and buttons etc.
Source is quick: Because it is lightweight it is quick. This is true in Edit and Preview and Publish modes (as well being lighter on the published page).
Source is free: Until now, on top of the cost of RapidWeaver and Stacks there was also the need to buy a framework. These all cost around $99 which is a substantial additional cost when starting out. Not any more. Source lets you start to build stunning freeform web pages without spending another penny!
Source loves third-party stacks: Let's face it, no framework has the best slider stack or the best gallery stack or the best accordian stack etc etc etc. For all of these things there are specialised stacks that have been crafted by third-party developers. There are over 800 of these stacks currently available over on the RapidWeaver Community website. Why limit yourself within the confines of a full framework when you can pick and mix the best stacks to build your sites?!
Source is powerful: Source comes with its own set of 8 stacks and even with only these you can build some really great sites. There is also a completely optional add-on 'power pack' that gives Source even more power of its own. Check out some of the example project pages to see the kind of thing that is possible.
So, that's my story and how it led me to putting together Source.
If your framework of choice meets your needs then great. But if any of my story rings true with you then do check out Source and see if this 'micro' framework approach might work for you.