While never use Batman during the show's run, it actually benefited the series because it made way for Justin Hartley to come in as Oliver Queen, a.k.a. Green Arrow. Back in the 2000s, the comic book media landscape was much smaller, and networks and studios had to walk on eggshells around superhero IPs.While Smallville introduced multiple iconic DC heroes during the show, there were a few that Warner Bros. never gave the show's writers access to, since they were prioritized for the movies. Of these, Batman is the most important.
Starting in 2001, Smallville told the origin story of Clark Kent, played by Tom Welling, before he became Superman. The show ended in May 2011 after ten seasons on The WB and The CW, making it the longest-running comic book-based show to date. Smallville was possibly the best superhero tv series ever. Its pilot set the record as the highest-rated series debut and the show went on with a loyal fan base for a decade, before moving into syndication and then streaming on Hulu. The Smallville creators have shared stories over the years about how hard they tried to get permission to use Bruce Wayne as a character in their show. But since Warner Bros. didn't the idea of having two Batmen in live-action at the same time, it led to Smallville making one of their best decisions ever.
Instead of the Dark Knight, they brought in the Emerald Archer, Green Arrow, with Hartley becoming the first actor to play him in live-action. After a recurring gig in the sixth season and a guest spot in the following season, Hartley was promoted to series regular for Smallville season 8. Green Arrow's biggest media presence before Smallville was in the Justice League animated shows, so the Superman prequel demonstrated his character potential to mainstream audiences. Batman had already been depicted so many times, and his character wouldn't have added many new layers to Smallville, besides Clark and Bruce working together. But with Oliver, the series had the best of both worlds. Green Arrow's dynamic with Welling's Superman became one of the better relationships in Smallville's later seasons, and it wouldn't have happened if they had had permission to use Batman.
While Arrow, had it not been for Smallville's decision to add Oliver to Clark's story.
In the end, Smallville did get to have its cake and eat it too, with the Smallville Season 11 tie-in comic finally introducing Batman. Even if it would have been significant to see Bruce and Clark on Smallville during its run on TV, adding Green Arrow instead was an inspired choice for the show. Having Clark befriend an already established superhero while still figuring himself out allowed the writers to add a different depth to his Superman origin story. Despite WB's Batman restriction being what it was, Smallville also gave Green Arrow the exposure he never had, which in turn led to Arrow and then the Arrowverse.