Samus's up-smash does not give Samus any upward KO ability, but at the same time is useful for comboing. Changing it to have more KO ability would sacrifice its ability to continue combos, but keeping it the same would leave Samus without the ability to KO upwards. Therefore, I see the addition of the stance change system as a logical third option, as it allows the preservation of the old abilities while still giving Samus an improved array of options.
From what I've seen, the extra knockback in ice mode may be used for easier KOs, but at the same time, it may be used for a better keep away game that befits a zoning character. (Theory crafting: ) Since Ice!D-tilt knocks opponents horizontally, it can be easily followed up with a super missile, or it may be used to pressure/punish by knocking opponents into an already fired super missile, or even on a platform, a homing missile followed by a drop-down super missile. One thing I have noticed is that super missiles completely miss short characters, and Fire!U-smash (most likely) completely misses them on ground. Ice!U-Smash is a scoop from the feet up, meaning that it can hit shorties, which may be used in conjunction with a DACUS as a punish for ducking a super missile (or maybe scoop them into said super missile). So logically, Samus does not have to be balanced around the stance system if the stance system is already balanced around Samus.
But then again, I have no Smash experience whatsoever, nor have I even played Project M yet, so if it appears I have no idea what I'm saying, I apologise. *bows deeply*
Lastly, I would like to ask, is it possible to crawl-tilt during a SWD? I can foresee its usage for some momentum mind games.