Well do elaborate on the subject,
I can't say I know too much about popular third generation pokemon. Though I've played the games, I'm unfamiliar with what promotes widespread importance/popularity these days.
Uhm I suppose I could start with talking about where the "popularity" this duo has gained came from. Basically I think the whole Pra_Mai crappy rumor, mixed with the idea that they could be the recarnation of Pichu really is what had most people sold on the idea of them being playable. Like I said earlier, most of us here don't believe that Pra_Mai is Plusle and Minun, but my 2011 poll was up during last year's E3 hype boom, and it was also placed on sites like Y!A, and Deviantart. So, the "less educated" speculators could have simply taken the rumor to be true, or just like the character, and added them. OR, someone voted for them a million times (Which would be extremely annoying with the way I have my poll set up, so I doubt that)
What makes Pokemon important/popular these days? Hmmm, for starters, Pokemon with extended air time on the anime, in a movie, one who is a starter, a legendary, or an event Pokemon have an automatically "Higher" status than any other Pokemon. Specifically Generation 1 Pokemon. If I was to bust out every Pokemon I think would qualify as important or popular, they would mostly be from Generation 1, and the Latest Generation. As for the third Gen specifically, I would consider the Starters (Treeko evo chain, Mudkip evo chain, and Torchic evo chain) to be pretty popular (Often coupled with the idea of May being their trainer), are very popular. Along with the legendaries like Deoxys, and to a much smaller extent, Jirachi. Gardevior has simply been a fan favorite, as well as a "furry esque" sex symbol, so she too is/was a popular choice. Lastly we have Plusle & Minun, the Pikachu's of the third generation. They're fame was previously explained. So there you have it, Starters, Deoxys, Jirachi, Gardevior, Plusle and Minun.
What significance would Pulse & Minum add to the Smash Kingdom?
Like I mentioned before, I see them playing at the "Proper Pichu". Something that didn't kill itself, and wasn't COMPLETELY a clone. But that's just me. They would function like IC's meet Pichu. Apart they would be weaker, but together they're a serious force. Moves like Iron tail, and discharge would be added in along with probably quick attack and pikachu's thundershock.