eamon
Explorer
Imo fun in an rpg comes from playing an alternate persona in a fantastic world and overcoming challanges (by your own, no matter what those challenges are), not by simply slaughtering higher and higher level enemies while your DM makes sure that you always have a chance of winning by direct combat.
I agree - but using a party's weakness doesn't necessarily mean exploiting it. As Tallifer said, you could let them confront their weakness and force them to retreat and rethink, or you could drop hints (perhaps the dragon's flyby strafing attacks are the stuff of local horror-stories). You can use their weakness to encourage some creativity or some changes in tactics, and mixing things up now and then is fun.
In a sense you have have you cake and eat it too - you don't need to tailor the challenge type match the PC's, but you can still have a party that always has a chance of winning - if they take into account what they know about the situation and plan appropriately. The world can stay reasonable, and the party won't get TPK'd by bad luck or an oversight. Ideally ;-).