I think all free rewards are interesting. And there are a lot in the game, happily.
But Trainer's challenge it's more than for the reward, to start. To start the game, and to learn to play after the tutorial, which is indeed quite short and lacks a lot of "tricks and rules".
Lets be honest, the IA is not very "I". Not even smart I'd say. The biggest challenge is to play the HGSS era Theme decks that can make a maximum of 80HP per turn, after suffering the inconsistency and get the only Stage 2 of the whole deck needing 3 or more energies with no energy accelerations nor any good supporter, against some EX the trainers have.
But after that, most of the times, the full charged EX jusr doesn't ever become the opponent's active, or they just use the 1st 30HP attacks, or just pass the turns without attacking!
Most of us don't even play more than 12 trainers with most of the decks. Just to get the pack, that's all. And without extra packs is going to remain that way, more than probably as better rewards are in Ladder and Events, which means business because you need good decks, and GX and so on.
The Trainer's challenge is there to help starting. Some coins and some packs. After that what they want is you play Theme in VS. And after getting some rewards in Theme, then go to Expanded and Standard. And buy packs, and collect cards, and spend money in codes, in IRL boxes, packs and bundles. That's the business!!
Playing Trainer's challenge is not business for TPCi so I don't think they vary a lot what we have.
And honestly, there's a ton of free rewards in game to add more on the boring+no business Trainer's challenge that almost anybody plays a lot. And if they almost removed Legacy, ruined the Event rewards doing it random, are not able to update the Starter decks from XY (red, blue and yellow) making something new with S&M era and the around 850 available cards with the FLi new set... they are not thinking into improve the Trainer's challenge.
Could be cool, but sadly, I don't think will happen.
WE WANT LEGACY TOURNAMENTS BACK !!! If you also want it, show your support http://forums.pokemontcg.com/topic/55966-feedback-making-it-easier-to-get-the-cards-you-want/
5th day reward packs list http://forums.pokemontcg.com/topic/56679-5th-day-log-in-pack-is-random-got-a-gur-dd/?p=593037
Foro en español, también existe: http://forums.pokemontcg.com/forum/2-español/