I think its like anything in life, lot of times you can do a perfectly good job with basic things, but there are often nicer options which make things more enjoyable or easier. It could be anything - like cars or woodworking tools. A cheap car still gets you from A to B, but a more expensive car may get you there faster, with more comfort. Better quality tools are often more pleasurable to use than cheap ones.
A CW filter makes life a bit more pleasant but is not an absolute must. I prefer to use a CW filter, but I also have a pretty well trained brain filter also. My brain was trained using RockMites -
The 300Hz filter in my FT817 was well worth adding, it certainly enhances my enjoyment of using the rig on CW.
73, Colin