There a two main reason why most timbers need to be stablised when used as reel seat spacers.
All timber expands and contracts to the changes in moisture and humidity which is normally not a problem the larger the timber object the more you can see this happen.
In all good woodworking skills joinery ,cabinet making construction of an item need to be produced to allow this to happen without defects taking place.
Many item of furniture now produced at cut prices made over seas do not take this simple task into account. (and in my line of buisness I see this week after week) unfortunally its the same with reel seat spacers and yes we have seen this problems with a very well known company in the UK (reel seats splitting down the grain).
Because timber expands if its is glued to a surface which does not allow any form of movement to take place like a carbon blank the reel seat can split.
Some timbers are more prone to this than others, the safe way is to stabilise this prevents this happening to some degree.
Surface finishes like paint and varnish do not prevent this happening to the same level of protection as impregnation, moisture can and will still enter the timber, you can see this happen in your home year on year as the window jams shut in winter or the door catches at the bottom but corrects itself in summer.
Most good makers of reel seat spacers will use some form of impregnate just to be sure. Its up to you?
gary
Nicho Bamboo Rod Co