it's interesting that SBW has been named as "midfield back" on teh blues website, whereas nonu is "second five-eighth", aumua and telea "centre". I'm probably reading more into this than i should but maybe that means they see a nonu 12 SBW 13 combo as an option.
i think nonu will be there more for a leadership role, particularly off the field. telea, as far as i can tell,is a really modest kinda guy with a good work ethic and would really lap up any lessons he can get. maybe aumua is more the type that could benefit from being humbled a bit and would benefit form a squad where everyone as working hard so its cut and dry that that's how it's done. (disclaimer: i know little about these people, and apologise if i'm wrong and happy to be called out on it).
in summary, i'm saying a Nonu at the end of his career just adds to the professional role models along with SBW, and could be good for the young centres in particular.
in terms of whether he starts or not, i'd say they'd want one of SBW and nonu starting (which will be SBW except if he's injured or rested/rotated) with one of the young guys next to him. nonu on the bench probably if SBW is starting.