Mary
What do you sugest the authorities mandate when it comes to flight manuals ?
You are looking at the situation from the glider pilot point of view, gliders very rarely go anywhere apart from flying around on comps, your take off performance is dictated by the tug or winch ( except on those tubo things with lawn mower engines) so for what you do carrying the flight manual is not very useful.
Now put yourself in the position of a flying club member who has hired an aircraft to go touring, I would think you would want to take a look at the flight manual when about to fly your wife & kids out of an airfield near Madrid with an OAT of 35 C.................. You might even decide not to fly !
The only way to make sure that the pilot of an aircraft has the information to fly the aircraft safely is to mandate that the Flight manual stays with the aircraft, this may be impractical in some aircraft but I can't see another way of doing this. It is not because you air expected to consult the manual in flight.