You do need both.
The persistent undo is great to be able to back in time, even when you
unload the buffer, quit Vim and come back another day. I especially use
it to add debugging commands, try lots of things, and finally just undo
the changes. No version control system needed.
The swapfile is a safety net for the unexpected. Vim might crash. I
know, for normal users this hardly ever happens, still I keep fixing
crashes all the time (fuzzers try just about any random input to look
for these problems). Or your computer might die. Hopefully that
doesn't happen either. But still, it happens when you least expect it.
Living in Hollywood is like living in a bowl of granola. What ain't
fruits and nuts is flakes.
/// Bram Moolenaar -- Br...@Moolenaar.net -- http://www.Moolenaar.net
\\\ sponsor Vim, vote for features -- http://www.Vim.org/sponsor/
\\\ help me help AIDS victims -- http://ICCF-Holland.org