To properly do a 2.5" lift, you need to run longer shocks and caster correction. You can argue until your blue in the face that it isn't necessary, but in my book, modifications should be done properly and safely, so those items are necessary.
Your argument that OME doesn't list the parts as a kit and separately is a red herring. Lets look at a few other things you buy for your vehicle that are done together, even though the parts aren't sold together:
1) Tires. When people buy new tires, they also get new valve stems and have them balanced. Sure you can put them on without doing so (and I'm guessing you do), but I would consider the stems and balancing necessary.
2) When you change your oil, the parts place doesn't sell filters packaged with the oil. Most get the filters because they are a necessary part as part of an oil change. Sure you could choose to skip the filter and let it go into bypass, but that doesn't make it a good idea.
There is a right and a wrong way do do things. Using old shocks that are to short and don't have adequate dampening puts you, your passengers, and those on the road around you in danger. To me, that clearly removes it from the optional list.
