1-The cost of a good duplexing scanner, like a Fujitsu ScanSnap at $400, or
2-The incredible amount of time feeding and turning a simplex scanner and
3-The need to cut bindings off bound material, or finesse them into a book scanner and
4-You hope you've got power when you need to access those manuals, and the printer so you can take a page back to the engine
Great idea, but it won't be cheap
or fast to scan everything. And of course, you'll need backups of everything in case your computer goes south. Or walkabout.