One thing to bear in mind is that some of the Sims 3 expansions have content you can only get if you buy digitally rather than with a disc.
For example, Sims 3 Pets has an additional 'cat and dog' costume pack you can download if you buy digitally. Additionally, the digital version of the Sims 3 University Life has the School Spirit Pack, Sims 3 Supernatural has a Plants vs. Zombies decor pack, and Sims 3 seasons has a Shark costume (for all ages) and a bumblee costume (only for children). I think Sims 3 Showtime has a simple radio if you get it digitally as well.
One reason you may want to look for physical discs is that you can still find some items that have Limited Edition content (the only way you could get the Limited Edition digitally is if you had preordered it via Origin). Sims 3 Seasons Limited Edition included the Ice Lounge, Sims 3 Supernatural Limited Edition has some Plants vs. Zombies material (the peashooter plant is probably the favorite), Sims 3 Pets Limited Edition has a pet shop that you can place, Sims 3 University Limited Edition has the Party Pack Bundle and Sims 3 Showtime Limited Edition has a new venue and stadium to play in (if you purchase the Katy Perry edition, the limited edition will be included with it.)
And if someone really wants to get all the content this late, then it will require making two Origin accounts. One would be with the physical discs registered, while the second one would be to repurchase the ones mentioned above for the Origin only content. And yes, this would get expensive in a hurry, and is not recommended except for the completist. As far as I know, EA has no intention to release the Limited Edition stuff as a separate purchase...