I would like Driveclub and Watchdogs to be remembered as quality experiences, rather than those "meh" launch games. The PS3 launch titles were atrocious, and look where it is now. Arguably the best library for the prior generation. Launch games do not matter as much as people believe. It's about quality distribution over the products life. They do matter, but they don't matter that much as long as they flow well after the launch.
Yeah. No point in rushing for a game to be complete for launch if it's not ready. Better to leave a lasting impression (especially if its a new IP) than selling yourself short to pursue some sales at launch. Plus it helps pad out the games better.
Though for Ubisoft.. I think its more of business decision, if anything. Releasing both Watchdogs and AC4 in the same month would eat into each other sales. It would make more sense if they were to delay the unproven title, pretty it up and make it better than releasing as is with something like Assassins Creed.