Some of these cards can change which side is in use once they've already been played, via the Transform mechanic
You cant just go online, add a bunch of sheets to your cart, and accept a shipment for a few dollars per sheet
It will also combo kill people out of nowhere later in the game
Sets That Contain Trainer Gallery Cards Dream League [SM11b]the only Gen 7 set with Trainer Gallery cards Skyscraping Perfection [S7D] / Blue Sky Stream [S7R] VMAX Climax [S8b] Battle Region [S9a] Dark Phantasma [S10a] Incandescent Arcana [S11a] Additionally, two Trainer Gallery cards were released as promotional cards while two others were part of the Ash vs Team Rocket Deck Kit [SMD]
While its true the majority of basics are nearly worthless due to their pervasiveness, a handful of basic lands have made their mark on Magics economy by virtue of being very rare, and very expensive
Its unlikely to affect competitive formats, but Commander players are going to absolutely love it