That's a new (and rather nice IMHO) feature of the Git and Subversion source control implementations. In addition to the raw source code you do version the PBL files. You also version the PBW file as well, which is also new.
You'll still be working with the raw source code files under the covers when you are doing edits and commits. So you're not going to end up locking PBLs and keeping other developers from using them.
The main reason the PBW and PBLs are in source control is so another developer an do a "checkout" from source control and get everything they need to get started working with the code. With the old MSSCCI interface, raw source code was the only thing that got versioned. So when a new developer did a "checkout" that's all they got, raw source code. They would either need to get a set of PBLs from another developer or generate the PBLs from source code. That's no longer necessary.
