It's totally up to you — your choice.
To me, when you have a theme enabled other than "TwentyEleven", "TwentyTwelve" or "TwentyThirtee", why do you need them? Especially when you have limited server space issue, and where every kilobyte is important. (But the server space for a Theme is too minor as to be a non-issue. — said Saildude)
But as you said,
Or should I just update them and keep them there for possible troubleshooting in the future?
Yes, sometimes, we recommend trying any default theme instead of the active one to find a bug. But you know, if you have the cPanel/FTP access, you can upload 'em anytime.
And for Security reasons: I don't think a deactivated theme can be a cause of site hack, especially those are provided by WordPress.org. But you know we are talking about the default themes and default themes only - for the others, I have no opinion.
So, to me, it's totally depends on WHAT DO YOU WANT.