Umm... you just proved the point. If he sets the respawn time to 1 year, or never (there are mods that do this), the cells may never respawn, and the data will remain in his saves forever, thus increasing the size over time and making it unplayable, like Tyrth said.
I agree that the data will then remain the the save forever and increase the size of the save over time. But what I'm saying is that more or less the same happens with a short (3 days, or even less) respawn time.
I'm rather interested in this discussion, since someone recently told me that changing the respawn time to 1 day, going in game and waiting 24 hours, then resetting it to another period of time (like 5 days) wouldn't actually do anything, since cells only respawn when you enter them.
That's what I'm saying, or rather that cells only respawn when they get loaded. Interior cells mostly only get loaded when you enter them, while exterior cells also gets loaded when you're maximum two cells away.
Anyway, the moment a cell respawn, most data gets cleaned out, but at the same time, new data is generated from all the spawn points and respawnable containers in the cell, thus more or less keeping your savegame size increased.
I haven't tested this personally, so it is possible that some specific types of data gets cleaned out earlier, but if I'm not completely mistaken, the two modern spawn mods, "Less Predictable Respawn" by Tekuromoto and "SPAWN" by shademe, both works on this principle, that you control the respawn of a cell, by changing the respawn time just before you load the cell. If cells got reset before they got loaded, this wouldn't work.