Because you might live in an apartment as opposed to a house. Because if all you can hear is bass floating around your room your decision-making process will be compromised. Because it may not be mandatory for the style of music you write. It really depends on the room.And the idea that you shouldn't have a subwoofer because it's producing frequencies you don't want to deal with makes no sense to me. Why would you not want to know whether you're sending out unwanted thuds and pops below the cutoff of your other drivers?
Now, subs *can* have different problems when they're not integrated well as part of a monitoring system. But that's a totally different issue from rejecting them because they produce low frequencies.
Either way, I’d argue that adding a subwoofer at a later time is pretty easy, and since I’m trying to stay within the practical realm of addressing what the OP requires, I would indeed add it at a later point. But hey, it’s a subjective decision for everyone and that’s totally fine; your point is quite valid.