That is true. Hugh barely battle you for 80% of the game.
I liked the huge amount of battles against Cheren and Bianca in Black & White.
I would have liked one more rival besides Hugh in Black 2 & White 2. We had Cheren and Bianca before, why not a boy and a girl again?
Actually, Hugh will battle you a good six or seven times over the course of the full game. This's roughly the standard for most rivals; Blue, for example, also fought you seven times during the story, as did Cheren; Silver, meanwhile, fought you six times (five during the main game, then once more at Mt. Moon), as did Barry. The outliers are Bianca (five), Brendan/May (up to four in Ruby/Sapphire and five in Emerald), and Wally (two, though one might argue that he simply picks up where Brendan/May left off, for six or seven rival battles total). Note that I'm leaving off the rematchable final teams in these numbers out of fairness for Brendan/May, who lacks one.
The likely reasons that it feels as though you don't battle Hugh as much are many. For one, his battles are largely loaded towards the back end of the game; you go a full five gyms--roughly half the main game--without facing him, from before Cheren at Floccesy Ranch all the way to after Clay at the Pokemon World Tournament, and the final battles take place in the post-game! Building on that, this is a fairly long game, particularly unlike the gen 2 games; you have the same amount of rival battles spread out over...I almost want to say nearly twice the time and distance. There is also the fact that you see him many times without him challenging you; contrast with, say, Blue, who fought you very nearly every time you saw each other. However, this only serves to make him the
most visible and active rival, rather than least; exactly how many times does he go into battle alongside you? Quite a few, right? About five times, I believe.
And then there's the matter that you brought up: that we're back down to just one rival. And let's be honest: Black/White kind of did spoil us for rival battles; one could be confident that when they reached a new city or got another badge, yet another challenge would be waiting just around the corner. However, it did not spoil us for actual boss fights--assuming you count rival battles as boss fights, of course; I do. In fact, part of the reason I'm perfectly okay with N and Ghetsis being the only Plasma leaders we fight in BW is precisely because of the twelve total battles with Cheren and Bianca. Conversely, the reason I'm okay with Barry only getting six battles in Platinum is because of the eleven battles against Galactic's higher-ups, and the reason I'm just peachy with Hugh only getting six or seven battles is (besides how many times we see him in battle regardless) because of the three battles with Colress, the four(?) with Zinzolin, and the numerous battles with the Shadow Triad, not to mention the loads of optional post-game bosses.
I got nothing on the exact reason we're back down to just one rival, but I'm pretty okay with whatever so long as the sum total of miniboss and boss fights remains roughly the same.
And as it turns out, them not being abrasive, self-righteous douchebags makes them more endearing as characters. After all, the main games have gradually gotten a little more plot/character intensive as the gens have improved; it hasn't just been the gameplay and Pokemon selection, after all.
Indeed. In fact, I'd argue that the tendency towards friendlier rivals is actually a direct result of the creators' desire to create characters with actual character arcs and development. These things generally imply something that's positive as a whole--the character working to become a better person, or at least garnering more sympathy from the audience. It's theoretically possible for a character to "develop" into more of a douche--a worse person--but such a thing is more typically referred to as regression rather than development, it does not generate the same feeling of progression that one gets from positive development, and even when it
does happen, it's traditionally treated as the lowest possible point for that character, before
actual (positive) development occurs (assuming the character is not a villain). And if it's done badly--which, more often than not, it is--the end result is stagnant, one-dimensional, and generally just kind of terrible.
Furthermore, I believe that the writers also have the desire to give us different rivals. And not just different in that they have different sprites and names, but also in that they have different stories to tell. Blue is your traditional boring dickhole. Silver is a troubled youth who ultimately learns that being a dickhole isn't a good way to go through life. Brendan/May only wants to help Professor Birch with his research and is about as casual as can be. Wally is the ultimate underdog story, the sickly small-town kid with big dreams who manages, against all odds, to step up and become your final challenge before the biggest of the big boys. Barry is a hyperactive but nice kid who dreams of following in his father's footsteps as a great trainer. Cheren starts his journey with huge dreams and learns that being the best isn't all there is to life, while Bianca starts off just wanting to have fun and ends up finding herself along the way, and both discover the true nature of their limitations. And finally, Hugh starts off on a quest for vengeance and learns that the Team Plasma of "today" isn't the same as the Team Plasma from five or even two years ago--essentially, that people, and the opinions and beliefs that those people hold, can change.
Honestly, I kinda believe the writers realized that there are only so many ways one can write an unsympathetic character, especially one who's not an out-and-out villain, and that there's just a whole lot more potential for nuance with sympathetic and friendly characters. You want an unsympathetic douche who remains an unsympathetic douche? We got one already, and Blue is just about as boring, stagnant, one-dimensional and generally objectively terrible as such a character can possibly be.