I only own the combo and it is plenty for me, but it depends on each persons situation. I don't play any gigs where I need more than the combo without PA support, mainly because I own plenty of PA stuff, and also work with plenty of PA stuff, so I am not short on volume when needed. The only time I need to be dependent on the combo is at rehearsals (which I am not rehearsing at present) and Jams, and so far I have not needed to squeeze the combo at all.
Having said that, if I had the space, I would most definitely pick up the extension cab, not because I need it but because it looks great and sounds much better than it looks