If I am honest, I found hardcoe mode in FO:NV to be more bothersome then anything... it's just a nuisance rather then an experience. It's just gone to that point of slightly too micromanagement like, having to constantly watch your hunger/thirst/tiredness and then your character still gets ill every time you fast travel... if people want it, then I am not against it as an optional mode. However I don't think it's worth detraction from working on better things if it will require a lot of work to implement and I certainly wouldn't want achievements/rewards based on it like FO:NV since that's pretty much going to make people play the mode who don't want to play it...
If it's a nuisance or not really depends on your interpretation of the system though. I know I'm one of a minority because not many people liked the hardcoe mode on NV but I personally liked it a lot. Anyone ever seen that picture of the Pip-Boy that said "Carry three guns and 1000 rounds for each" or something like that? I always thought that was kind of stupid. As well as taking a nap would heal you and fix up your crippled limbs. hardcoe mode is really only a nuisance if you get into the mindset that all your meters have to be at zero at all times. The way I played (Still play, In fact am off the forums to go play some soon) hardcoe mode is; ;if I cripple a limb and don't have a doctors bag..man up and fight with a broken leg until I can get to town. or if I'm dehydrated without any water on me "Dehydrated? Eh..only minor dehydration. I'll drink from a toilet if I stumble across it". Get hungry? deal with it until you get a chance to kill something and cook its meat at Goodsprings Source. Besides on FO3 I constantly found myself wondering what the point of all the ingame food was since I couldn't cook with it and eating it did nothing except for a few minor, short term buffs that drugs could easily accomplish, but much stronger..
Maybe that was a bit wordy but my point is proven. ;D hardcoe mode is a nuisance if you're looking for a quick play. If you're looking for a dedicated RPG where what provisions you choose to take on a journey with you may just save your life, and running out of ammo or food is actually a concern it can seriously enhance the experience.
Edit: Granted it should be a choice.