Do you need one, no.
Will you struggle to go back to a cropped sensor once you’ve used full frame, yes.
The high ISO performance on something like a 5D means I can get shots where I wouldn’t even bother getting my 550D out.
That said, I’ve been on the cheaper Canon’s ever since they brought out the 300D and still have shots that I consider keepers from that very first body. But the 5D I have at work is something else, but then it’s easy to justify one when it’s for commercial use.
Size is a good point. I do a lot of DSLR video (more than stills in fact), and when I’m out riding and filming, I don’t even entertain taking the 5D due to its size.