Really? I mean, I can understand, but Pichu is a veteran. I believe Pichu, as irrelevant as he is, deserves a spot in PM if only because he's a veteran. Jigglypuff is incredibly irrelevant, but if she got left behind, there would be far more outrage because she's one of the original 12. Pichu is more relevant if only because he's related to Pikachu, and he's a veteran, and even though he's not from the original 12, he's from the game Project M was created around, and in my opinion that should more than reserve a spot for him. Also, with the Pichu Bros., you're getting an iconic, memorable anime characters, and in the process bringing back a Melee veteran that a surprisingly large amount of casual people like, but revamped to be competitively viable so competetive players like him as well, and representing an underused character archetype that some people like: Two-in-one characters. And, by giving him the ability to SoPo, Melee fans can play as regular Pichu. It makes far more since than a generic Plusle & Minun because the Pichu Bros. are rather well known, takes up less time and is easier on the PMDT due to easier model editing for Pichu Big and less voice clip space since they can share, and for PraiMai fans you can include a Plusle & Minun-like palette swap/Alt. Costume cosplay. Everybody wins.
I understand your dislike for Pichu. In Melee, I liked the character, just not the way he was implemented. I really want Sami because we can represent an obscure series and put in a really sexy cool character, but Fire Emblem has 3 characters as it is. Lyn, while cool, just doesn't live up to the potential other characters can. In my opinion, four FE characters in Sm4sh is pushing it, and three is just right for PM. I don't think we need another FE rep. And, yes, Pokemon is an very over-saturated series, but adding Lyn makes FE saturated, especially since Lyn brings almost nothing new, unlike the Pichu Bros. I just feel like the Pichu Bros. can bring more to the table due to the whole Two-In-One deal than Lyn, can appeal to a far larger group of people, and would be easier on the PMDT due to an almost completely duplicated MoveSet, a complete set of voice clips that can be shared, a simpler model (PMDT said they wouldn't use AT models), and overall more importance due to their roles in the anime and in Melee, and the vast sea of duplicatable animations.