I rely on project files for TextMate for the following reasons:
(1) I work on many different projects, bouncing between the sets of sources within each project. By opening a project file, it makes it easy for me to restore where I was at when I bounce from one project back to another.
(2) I work in a team, where each developer uses different tools on different platforms. I prefer to keep my TextMate project files in a completely separate area away from the sources so I don't have to worry about polluting the source repository with "my baggage."
(3) Because each set of sources contains multi platform files, I use the TextMate project settings to hide sources (via regex syntax) for files I don't want to see or more importantly find references in when I do a "find in project" search.
Please keep projects in TM2 or an equivalent way to group sets of sources.