There are currently a number of issues with DXL round-tripping that affects use of source control. These include:
I discussed this with Andre Guirard at Lotusphere 2014 when I (Paul Withers) also presented a Show and Tell session with Declan Lynch. The impression I got at Ask The Developers was that there are other issues I hadn't identified. I know DDE does something to extract Java agents as a separate "Project" when you edit them, so I'm not sure how DXL round-trips those. When analysing DXL for design elements recently, I've encountered failures to read the DXL sometimes, possibly related to older databases. I believe there may have been some improvements around DXL for or in ApplicationInsights, I don't know what they were or whether they're in recent Domino or Domino Designer versions.
WHY IS THIS IMPORTANT NOW?
With the news about HCL Nomad there has been an increased focus on traditional Notes Client applications. But the issues identified and potential for others makes this an inadvisable route for source control management. Other proprietary solutions are limited to Domino, meaning differing source control tools and experiences for Domino and non-Domino solutions. In the current landscape and the desired future for Domino, that also is not desirable.
THINKING BIGGER, OPEN AND DEVELOP AS ODP
Source control requires single flat files rather than the Design Notes in an NSF. I don't know if that can be addressed. It's not beyond the realms of possibility that opening a database in DDE could actually generate an ODP or allow conversion to an ODP, and an "Applications Navigator" could launch the simulated, "Views" of design element types (Forms, Framesets etc) generated based on metadata, with the appropriate editor launched automatically.
BENEFITS FOR DEVELOPERS AND HCL