But there's no cost to it when I walk back & forth to Megaton/Novac/Whiterun again, through empty territory & no encounters.
Well except for my personal time. And I already will spend plenty of time with this F04 (as I have with several previous ones), so I see no need to pad out my gametime ("see what incredible value the game is? 400 hours of play for just $60" "But 100 of those hours were spent walking back and forth to your house. What amazing adventure.") with repetitive and mind-numbing tedium.
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Typical early-game adventuring for me, in FO3. Pick a quest far away in a direction I haven't explored - like going to Republic of Dave. Start walking there from Megaton. Get distracted by dungeon - explore it, loot it, exit. My inventory is full. FT to Megaton, deal with loot, FT back to Dungeon A. Continue walking towards RoD. Get distracted by Dungeon B - explore it, loot it, exit. Inventory's full again. FT to Megaton, deal with loot FT back to Dungeon B. Continue towards RoD. Repeat this many times, until I finally get to the Republic, several IRL days of playing later. Nothing in this experience would be improved by replacing each of those "FT to megaton/FT back to where I left off" segments with "trudge mindlessly across empty terrain, over an ever-increasingly-long route". In fact, it would likely make the experience much more miserable, as larger and larger portions of my play time each day was svcked up by busywork and repetition. Making me enjoy the game less.