MLS, we are (I believe) talking about the UK here - where the airfield density is rather greater than in most parts of the world, and where gliders have been historically deregulated and legally speaking maintained by owners using non-certiifed parts (certainly not the case in most countries where they hold ICAO compliant CofA). Also our fields tend to be rather smaller than you are probably used to in Canada, and often rather less suitable for a forced landing - this means that gliders often do divert to GA or microlight fields, where most of the time they display superb airmanship and are given a friendly welcome.
The mainplane and other composite primary structure is stressed to higher values than light aircraft to compensate for the variability of composite materials. The flying controls are stressed to the same or slightly lower values.
As for glider joins, I'm afraid that in my own flying I see this 2 or 3 times a year - which is a lot. Charts are available with radio frequencies on them and it is perfectly possible (I've done it more times than I'd care to count when diverting for one reason or another) to call an airfield when very close, get circuit details and make a reasonable stab at informing the airfield / other traffic of your intentions, and most of the time fit in with prevailing circuit traffic. It's not hard, and since in the UK most airfields are surrounded by 2nm radius controlled airspace, that's at-least 60 seconds from entering the ATZ to setting up for a landing, quite enough to announce your intention - and a damned good idea at a busy airfield which might have half a dozen powered aircraft, some with students on board, in the circuit.
I'm not anti-glider, I've flown them, certified them, and been very happy to welcome a great many to various airfields I've flown from. I'm anti bad-airmanship, and I'm afraid that gliders are not immune from it - nor is any other branch of aviation, but this thread is about gliders.
G