Originally Posted by
FBinNY
There's no reason to double up on the shims. They're longer than the minimum insertion depth so you'll get no benefit from the second, lower one.
Your post in a 100mm shim (3 diameters +) is the same as any post inserted 3 or more inches into any frame. These guidelines aren't pulled from thin air, and are based on the depth where close to 100% of the theoretical strength is attained. Or to put it another way, there's no gain (none, zero, zip nada) in support or rigidity if you go deeper.
...this makes little sense. A long seatpost inserted into a seat tube will increase the stiffness (rigidity) of the seat tube/seat post combination for whatever distance it is inserted. (If it is the correct size and slip fit in the the seat tube). If you are talking about something else, like the rigidity of the seat post and saddle position, that's a different horse color.
Other than not having to cut the post, it's a good reason for keeping a post long, if you're interested in further stiffening what is already one of the stiffer tubes in the frame.