When Nintendo made each Nintendo Network user create a new ID, it was to ensure cross-compatibility between all of their systems. It also had the added advantage of keeping all your purchased content on one account, rather than individual machines.
Now, another benefit has come to fruition, with the first cross-buy game for Nintendo platforms set to release on the 3DS and Wii U this July 3rd.
There are a few caveats, though; the offer is only available to Europeans and the purchase has to be made on the 3DS in order to get a free Wii U version, not the other way round.
The game in question is Squids Odyssey, a platformer which features turn based battles between squids.