Harrison, Tyson, and Takuto - They were there to beat Ash. That's all. They provided little entertainment value outside of their pokemon and Ash's pokemon beating the **** out of each other. All three have very similar personalities. Takuto seemed like he could be different due to the mysterious factor, but in the end he was just a mature, older rival too.
personally, I think Takuto was alright. Sure, it's annoying that they don't even try to explain how he was able to acquire legendary Pokemon (a league of monsters which, prior to Jindai's Regis, seemed downright uncatchable), and there's a very good chance we're never even gonna hear of this mysteriously powerful character ever again (though if we do, and Satoshi gets in a rematch, I'll applaud the writers very much). However, I like the fact that Satoshi was taken out of the league by a character that, for once, he had absolutely no connection to. Losing to a skilled opponent you're only meeting for the first time is hardly unrealistic (on the other hand, ALL the trainers who defeated Satoshi in tournaments prior to this were people he got along with after he just so happened to meet them immediately as the tournament was beginning). Also, considering the much-hyped rival battles he was having, if Satoshi was still going to lose the tournament anyway, it was a nice move to at least have him take out TWO legendaries in the process.
I'd also give Tetsuya at least a few points for having an awesome Pokemon (even though he himself was pretty dull). Not like they could've done much for that tournament anyway, since they couldn't even establish a long-standing rival for Satoshi all of Houen, and the only one they gave him a few episodes prior was rather inexperienced (Masamune). Still, given all I said about Takuto, it would've still been nice to not go the "friendly rival I just got to know" route for the 3rd time in a row. Kaede was an awesome enough opponent for him to lose against... in fact, just about everyone competing in the Saiyuu Tournament was infinitely more interesting than Tetsuya (seriously, if you were on those boards while those episodes were airing, you'd know that a lot of characters garnered some fans just for having an appealing design that showed up on screen for a mere couple minutes...
Jump,
Yuriko,
Hyuuga, etc.).
BTW Thunderblade, since you mentioned just about all the other rivals, regardless of whether or not they did anything more than compete in a single league battle against Satoshi, what were your thoughts on the Houen competitors (and I guess
Kiyo, who only turned out to be a drop-out just like Nanako before him)? And, for that matter,
Korm and
Kaoruko from the Sekiei Tournament.