My own take on the beta process is that devs on one side commit to adding and improving features, but also to ensuring that the new releases are, as far as reasonably achievable, free of bugs and that the new features aren't broken on release. To balance that we have a general policy to freeze the code when the beta starts, where code changes should primarily be fixes to issues detected in the beta. Exceptions are made on a semi-regular basis for changes that go further if they can be justified. That's mainly the case when there's still enough time left in the beta to get the changes properly tested and reviewed, or the risk of breakage is low i.e. because you're adding, but not changing things.
For example, I worked extensively on assets this release cycle and didn't manage to get all the work done before the beta, so I was still busy until about the middle of the beta phase. However, I prioritised my work so that asset changes were ready ahead of the beta, while asset additions (which won't break anything) were made during the beta. I also treaded particularly carefully when adding the new assets to compensate for their shorter beta testing time.
I might not have been around as much lately, but I only heard of Daft Mugi's slew of patches when we were discussing whether the next beta build should be the release candidate, which gets released as-is if no significant issues are reported. At that point any changes will either prolong the beta testing phase, or carry the risk of not working as intended. The plan was to reach this point at the end of january (if this was not prominently communicated somewhere it was a mistake).
Personally I definitely see the value in trying to accommodate as many of these changes as possible because they make TDM more attractive to Thief players, Daft Mugi is even standing ready to introduce them to the TTLG community, and it looks like they're fairly simple changes. However, prolonging the beta comes with its own drawbacks. One is that players and mappers are delayed in getting access to all the other new features. Another is that the beta phase is a relatively draining part of the release cycle for the devs because new projects are mostly on hold while the focus is on getting to a stable build. One of the ideas behind dev builds was to shorten the beta phase by spreading out the testing.
Ultimately, we've designated Stgatilov as the project lead so it's his task to call the shots on how to balance these demands.