I'd say realistic mechanics over player convenience. For me to become immersed in a game I must feel like my character is part of a living, breathing world rather than feeling like the entire world exists only to pander to my power-fantasy.
Morrowind was a game that was very good at immersion, because it wasn't afraid to let the player make mistakes or to hold the player back at certain points. Every NPC was killable, even quest specific ones, which could lead to situations where the PC had eliminated their chances of progressing down certain questlines if they had slain the wrong NPC. Inconvenient, certainly, but realistic. It was entirely possible to drop, sell or otherwise lose quest items (I remember once being sent to recover a holy artifact from the body of an Ordinator only to (re)discover that I had encountered him before when questing at a much lower level and had stripped him of all his gear to sell for drakes for training). Again, inconvenient, but by making the PC fallible the game impresses upon the player that their every action, even seemingly meaningless ones at the time, have repercussions. Advancement in the various Guilds, Houses and other organisations of Vvardenfel required actual skill in the requisite abilities and spheres of knowledge, meaning that if a character wanted to become head of the Fighters Guild then that character had to demonstrate that their abilities actually counted them amongst the best warriors in the land (requiring a combat skill to be in the 90s before they became eligible for promotion).
Compare the above with Oblivion, which was not very good at immersion because it refused to let the player make any mistakes. Any hint that the player was about to do something wrong would cause the game mechanics to come clunking down around the character, removing all immersion by forcibly reminding the player that they are, in fact, playing a game. Every NPC involved in a quest at any point in the game was unkillable from the off, resulting in the rather odd situation of making it seem like Cyrodiil was inhabited by a race of immortals. Great for not letting the PC put a foot wrong, but hugely immersion-breaking. Characters would end up lugging a backpack full of seemingly random items around with them, with no idea what they were doing with such a strange collection of gee-gaws, only knowing that if they tried to unload their inventory they would be told by some mystical being that "Quest items cannot be removed from your inventory!". Again, great for keeping the training wheels on the PC so that they can't f*ck up, but completely removing all sense of consequence from the game. And players were effectively power-ramped through the factions, as the NPCs immediately recognized that the PC was somehow deserving of special treatment and that their personal abilities should in no way impede their meteoric rise through the ranks. This would lead to very odd situations, such as a warrior being crowned Arena Champion despite only having joined up earlier on in the week. Very awkward when conversing with the other trainees who regale you with stories of how it's taken them ten years to attain the rank of gladiator. Again, perfect for PC power-fantasy fulfillment, but piss-poor at actual character arc development and a sense of achievement.
TL;DR: At least for me, more realistic mechanics result in a deeper and more rewarding experience.
