To me the srd3.5 was one of the main reasons to switch from AD&D 2e to 3/3.5.
Why? Because it allowed Copy&Paste and tools like PCGen to be created.
I have since bought about 60 books and the PCGen datasets for 49 of those from CodeMonkey Publishing.
I have gotten addicted to using PCGen to create my games, and have willingly shelled out several hundreds of dollars to enable the tool to legally use
WotC published materials instead of just the SRD3.5 material.
But PCGen would not exist without the
SRD, at least not in it's current form.
When
WotC made it impossible for CodeMonkey Pusblishing, to provide datasets for PCGen, I stopped buying books because I knew I would be able to use the material in PCGen. I am still very disappointed that some of the books published in 2005 and 2006 never were allowed to be provided with support for PCGen, Like the Magical Item Compendium.
4th edition is just not an option for me without similar options to create and modify characters, items, feats, skills, templates, trade them with the players and so on and so forth.
My point is, that the combination of CMPs legal status and the
srd 3.5 availabilty to PCGen allowed me to have the contents of nearly 50
WotC published books at my finger tips in an organised way. Without it, I would not have purchased as many supplements and would have stuck to the core 3.5 PHB/
MM/
DMG like, I did with ADnD 2e and I might do with
4e, if I even switch to
4e at all given how much more convenient 3.5 is to use.
Edit: P.S. Oh, and it runs on my Mac.
Edit2: And about the
DDI, I basically have very little confidence in
WotC's ability to support and develop software properly over longer periods of time given their history. The way they pulled support for e-Tools/PCGen still leaves a sour taste in my mouth. Do't they say: burnt once, twice shy?
I wish I would be able to get exited about 4th edition, I want to buy
WotC books, as I want to support them in developing one of my favourite games, but currently they're forcing me to stick with 3.5 and scavenge second hand copies of ebay for those books I am still missing. It also means my local FLGS hasn't seen me a lot lately.
Edit3: And the
SRD also made it easy to Copy/paste together a list of all the monsters I can summon and similar such things that are much harder if you only have a hard copy of a book.