The 1-day Paris Visite 1-5 costs 17.05 euros; the oneway RER fare is 8.10; a metro carnet is 10.70 euros. Unless someone is going to do a lot of metro-rer trips in one day it is probably cheaper in the long run to buy one way ticket and then when you get to central Paris buy a 10 trip carnet, which essentially has no expiration date.
A longer period visite could be justified if one were to do a lot of trips outside Paris spreading these over several days.