I kinda agree with Henry - I don't think that a player who wants to play a Swashbuckler is entitled to be as good a meat-shield-tank as the guy who plays a Dwarven Defender, **but** there needs to be give & take here. If the PC swashbuckler is going to forego wearing medium & heavy armour, a good GM will do what the DMG suggests and **work with the player** to tweak the character's abilities by eg swapping out Medium & Heavy armour proficiencies for something different and useful to a swashbuckler - eg give them Tumble as a class skill (revised Quint Fighter has this as an option, BTW). The Tumbling swashbuckler won't be as good a meat-shield as his plate-armoured buddy, _but_ he will have gained other abilities, he can now do things the plate-wearer can't.
The important principle: if the PC gives up a useful ability, give them a different useful ability in return. Don't give them new stuff without a cost (the unrestricted splatbook approach), but if they are giving up stuff for a concept, give them appropriate compensation. This goes for NPCs too - if the Egyptian-mythos clerics in your game don't wear any armour & never fight, give them d6 hit dice & no armour profs - but give them 6 skill points/level and a better skill selection.
The important principle: if the PC gives up a useful ability, give them a different useful ability in return. Don't give them new stuff without a cost (the unrestricted splatbook approach), but if they are giving up stuff for a concept, give them appropriate compensation. This goes for NPCs too - if the Egyptian-mythos clerics in your game don't wear any armour & never fight, give them d6 hit dice & no armour profs - but give them 6 skill points/level and a better skill selection.