Although I've never really fully incorporated encumbrance rules in a game or ever been terribly strict with my timekeeping or been very precise when it comes to movement speed (or ever known anyone else who was) I've always wanted to be. Those just seem like the kind of things a good game master should trouble about. It's rules like these where the game is it's most game-ish. With limitation of course - There's no reason to make a big deal out of that new magic sword you just found encumbering you or hauling away a mountainous pile of gold after the whole dungeon has been cleared out. But if there are still bands of gnolls prowling the corridors - that's another story. Same thing with light sources. There's nothing terribly fun about rolling dice to see if you get the campfire lit but if you are two miles underground and your last torch burns out while you're in the middle of fighting off some beasties, well that is fun. Or, rather, it might be fun provided the encounter ends well for the party. It's fun to me anyway but I've a cruel streak.
The same thing with encumbrance and provisioning. Delving into a mega dungeon should be a major undertaking. Troubling about pack animals and hiring porters or men at arms for the journey (despite being a big headache for the DM trying to track initiative and effects) should add depth to an underdark adventure. You would have the added enjoyment of managing your gold, your henchmen, and put a deal more thought into such an expedition than your average dungeon crawl. Again, it seems it would potentially add a great deal of fun to the game if you had a mule to sacrifice to a pack of hungry wolves to enable your escape or to eat yourself if you got trapped on level 3 or to haul away some great golden idol or to trouble about provender for the mule after three days underground.
All the same I wouldn't want to turn the game into hours of accounting and inventory and collecting rents but nevertheless I am going to start trying to be accurate with my timekeeping and light source radii and mileage per terrain type, &c. So I guess this is my warning of sorts. Make sure you have enough food if you want to try and journey over that mountain range because I don't want to spring starvation on you unannounced.
Of course all of the above depends upon me to actually provide the kind of scenarios/campaign where this kind of thing would even matter and hopefully make it fun at the same time.