because main .yyp file in some way duplicates data from resource .yy files, when you change order of something in tree or add a new resource, or even new resource tree view, and another person in your team do one of those at same time, you will have unsolvable conflict, since there will be one million new entries in project.yyp file instead only references to them:
If you change the order of resources in the tree it doesn't affect the main .yyp file, unless you've added a new folder/group to the project in the process - resource ordering information is stored in folder resources. However, adding a custom resource tree view does add a whole bunch of new folder resources to the project, iirc.
Ignoring "room_order" (which is obsolete and should be removed) and "script_order" (which shouldn't be necessary), the only duplicate data between the .yyp and .yy files should be that the "Key" of any given resource in the .yyp is the same as the "id" in the .yy file and that the "resourceType" in the .yyp happens to match the "modelName" in the .yy.
The block of information added to the .yyp for a new resource tells us what a resource is and what's required to load
all its data. Because most of that block tends to be the same between various resources (as it stands) it does cause source control conflicts, but for just one resource it's trivial to solve the conflict - "use text block from 'theirs' before 'mine'" does the trick. However, I've not tried anything more complicated and even having to resolve this type of conflict alone is not ideal (I know what the end result should be...).