(I'm writing this largely for Catholics who aren't ordinarily interested in most Protestant resources.)
It basically boils down to four main reasons that I can see. If you can see more, please add them.
1. Access to program features you may not already have, including everything supplied by Verbum/Logos Now.
2. A substantial collection of patristic works that goes well beyond the Protestant version of Schaff's ECFs.
3. Many reference materials are included that, although not of Catholic origin, can nonetheless be of Catholic interest.
4. Whenever else will you get a chance to look at a lot of Protestant resources and investigate whether you will find them helpful for free? (Also, several of Fr. Raymond Brown's books can be found in there, if you're interested in them.)
