The roster suggested by the leak seems totally adequate to me... seems strange that THAT's a premise on which people argue it's validity.
Because assuming this is truly it for the newcomers, and ignoring any glaring omissions of fan favorites, there's;
*A startling complete lack of villains and heavyweight characters (the former of which there has been an especially low amount of in smash, and were clamored for just as much as more females, so it would be odd to address the lack of females but completely ignore the lack of villains).
*A lack of older representation (outside third parties, all the newcomer revealed and implicated are those featured in very recent games or recently revived franchises; in fact 6 of the 10 non-third party newcomers actually debuted around or post Brawl's release).
*Such a low amount of newcomers being kept secret (it's extremely likely we'll get one more newcomer revealed before release, which would mean we would only have two kept secret, even less than Brawl's paltry four, which was one of the biggest casual complaints about the game).
Anyway, as for the recent subject of if this is truly it (again), while it's pretty clear the leaker had legitimate inside info, I think there's reasonable cause to believe there will be more newcomers.
*The aforementioned holes in the newcomer representation indicates to me there's a few more newcomers we're not aware of.
*It would just be uncharacteristic of Sakurai to reveal this much of the cast to us prelaunch, especially when as previously mentioned, that was one of the biggest casual complaints about Brawl, and even Sakurai said he wasn't going to release as much info about Smash 4 prelaunch as he did with Brawl, (which as far as we can see, has been held true, as the Miiverse updates have been notoriously less informative than the Dojo updates, and there are many parts of the games that are still a mystery to us while being so close to release; we probably don't even know half the stages yet). Even with no more characters being revealed before launch assuming the often thrown around 47 roster, we have already surpassed Brawl's prelaunch revealed proportion of its roster (and we're likely to get at least another two characters revealed before launch, making the proportion of prelaunch reveals even more lopsided). On the other hand, if we go with the other commonly speculated roster number of 51, and then get that likely two more prelaunch reveals, we would pretty much be exactly in line with Brawl's prelaunch revealed proportion.
*Even assuming it's all 100% true, the leaked information itself. Missing Rosalina still indicates there's more; sure you could say she was "inbetween the leaks", but if that was actually the case and the leaker truly has all the newcomers, what evidence is there that he definitely isn't holding more back, when he held back half of the newcomers he gave us during the first leak and continues to leak in spurts instead of all at once? The leaker's vagueness in spots/getting specifics wrong also shows his information is not absolute and up to date, nevermind the fact the leaker never once claimed to have released all the newcomers or even know the entire roster, and hasn't told us anything about which veterans will return/get cut beyond a single vague nondefinitive mention of a Ness v Lucas debate; surely he would have said something about the status of Snake or Mewtwo at least, or something about transformation characters going solo, if he knew the full roster and was telling us everything.
Even without going into theories like the leaker got a hold of E3 2013 plans and then guessed the rest, or is a marketing guy who is made aware of newcomers they plan to reveal prelaunch and hype up while never being made aware of the secret newcomers, I find a 50-53 roster more probable than the 47 roster people are claiming we're getting. But eh, we'll see soon just how much the guy knew and was telling us.