Darn -- If it isn't Michael Wood, it's Tom Welch that beats me to the post. ;-)
Project-based organizations of any size have to be matrix-based for full efectiveness. The alternative is purely dedicated resourcing of projects, which is a waste of talent and drives all kinds of self-defeating behaviors on everyone's part. No one skill (other than PM) is needed across the full duration of a project, so a matrix is, as Tom puts it "the default" structure whether you recognize it or not.
Shared skilled resources (managed and supplied by the "resource management" axis of the matrix), guided to appropriate priorities for their use at any point in time (by a combination of a coherent strategy by project owners and execution through PM on the other axis) add up to effectively managed project organizations.