First, let me say that I'd like to have either player on my team.
Call me crazy - but if Sejna was able to produce offensively, why did he only have 2g & 2a for 20 games? That would put him on pace to score 8g & 8a for a full season. He was also getting 15 minutes a night, which is pretty decent icetime for a rookie. Add in ALL of his points came on the PP, and I think that anyone could make a case that it was just Sejna's defense that got him sent to the AHL.
Also, the Blues were desperate all year for someone to step up and score on the 2nd line winger positions. Hell, they went so far as to give Brian Savage a chance (free or not!)!
I think that Sejna just needed some time to adjust to the pro game. I do think that it's a HUGE assumption though to say that he was ready offensively, but just wasn't polished enough defensively.