Adjusting the standing rigging may seem straight forward, simply tighten the stays until the mast is in column and in the correct order that way everything lines up and works well together. The problem is, how tight is tight enough?
You don't want to over tighten the stays because the additional stress is just that, additional stress. This stress is transmitted to the spar, the mast tanks, the chainplates, the hull, pretty much everything that is involved with the rigging will now be subjected to unnecessary additional stress.
There are two ways to check the tightness of the rigging, one is at the dock, the second is while sailing.
The dockside check is rather simple, all you need to do is feel how tight the cap shrouds are. They should be tight enough that when you push or pull on them, they don't want to jiggle around too much. If they feel loose or floppy, you need to tighten them further.
Once the cap shrouds are set, the rest of the shrouds simply follow in sequential order with none of them being tighter than the cap shrouds.
The second method, and the preferred method, is to sail test the rigging. This will subject the yacht to the real loads that it is expected to perform under. If the mast is not in column while sailing, the windward shrouds are too loose and need to be tensioned further. If the mast is in column but the leeward shrouds are floppy, the leeward shrouds need to be tightened to remove the slack.
Eventually, rigging perfection will be reached where the shrouds are the perfect tightness. The mast remains in column on all points of sail and the shrouds never go dangling slack.
The reason really slack leeward shrouds are a hazard are all due to the practical effects they can have on the rigging.
If the cap shroud were to slip out of the spreader tip, it would then be too loose and the spreader would no longer be working. The mast would bend violently to leeward as the overlong cap shroud is over stressed. This is why the spreader tip needs to be tied to the cap shroud, that way it won't fall out of place, even when slack.
The second reason really slack shrouds are a danger is made apparent during tacking and especially during jibing. The slack rigging is under no tension while on the leeward side. Should your yacht undergo an accidental jibe, the leeward shrouds would quickly become the windward shrouds in a violent display of force.
The slack shrouds will be shock loaded, and these shock loads can be tremendously greater than what the stay is able to withstand. An accidental jibe could actually snap a stay, which results in overloading the remaining stays and potentially bringing down the mast.
If the leeward shrouds are kept in slight tension, they will not go from slack to fully loaded in an instant and the rigging will have a better chance of surviving a fierce accidental jibe.