Spirit isn't a Mod.... but then again it is... Confused, so was I until I gave it a try.
I too, am surprised that more people don't use it when they build levels, the tools are great, difficult but great. If you have ever mapped for TFC then it isn't quite as difficult as that.
This is probably incorrect but hey? Spirit is a tool that unleashes the abilities of the engine to give you more control over entities. Problem is that to distribute your brilliant map, others have to have spitit to run it. So yeah, it's a Mod, but it isn't. It is just a way of doing things better. That's why I am surprised more people didn't make it the basis of their Mod's.
I am pretty sure that Spirit was used in DoD... and I think NS uses it, but don't Quote me... I have been known to be wrong. I wouldn't be surprised if it was the base idea for some of the stuff in HL2. Either way, you as a user/player of Mods made with Spirit don't really see anything different, unless it's really cool fog or rain, but behind the scenes heaps of stuff is going on. You can make entities move with other entities. A func_train can carry and have a useable func_tank... Guess what, you now have a moving tank that fires, and the turret rotates.
A Mod made with Spirit, does not require the user to have Spirit as it's all in the DLL, but if you are distributing standalone maps.. Then to view them properly, you need spirit.
Does that help?