There are a ton of problems with it:
(1) MKX was originally advertised by Boon as a new generation of MK, complete with a roster comprised of at least half new fighters. So far, we have 8 (Takada, D'Vorah, Ferra/Torr, Kotal Kahn, Cassie, Jacqueline, Kung Jin, Erron Black)...out of an over 25 character roster, excluding DLC (which will include 2 guest characters).
(2) If the parents are konfirmed, then that brings the Special Forces count to 7 (excluding Kenshi). As it stands, faction representation is horribly out of balance.
(3) Many would argue that the parents are actually some of NRS's least creative work. By including the children, it was the expectation of many fans that the new generation would be more complex characters that took the best traits of their parents while ignoring their flaws. Essentially, many people saw the new characters as a "reboot" of the previously lackluster design, but that has been diminshed by the inclusion of the lackluster characters.
(4) Moreover, while we don't know why the previously dead characters are returning (Kitana, Kung Lao), each reveal at this point is beginning to feel like MKX is moving further and further away from Boon's original goal of making the series fresh and going back to its roots (make a bunch of clone characters with different special moves). Cassie shares a shadow kick (combo ender), a nut punch, and a cartwheel kick (backwards flip kick) with her parents. Even if the move set is played entirely differently then her parents, why would you want to take up three roster slots with similar moves?
(5) The heart of the problem is a lack of creativity (or intentional absence of it). The inclusion of Cassie was suppose to free up two more roster slots for new and exciting ideas from NRS. Instead, we are getting rehashed (though some fan favorite) characters. Jax, Johnny, Sonya, Kung Lao, & Kenshi all have spiritual successors in this game. By including them, you have 5 character slots filled that could have been new creations (or other fan favorites that didn't already have ties into the game).
TL-DR: Having a character play different then another isn't the issue. Including the parents and the children takes up to many roster spots AND takes away from the promise of MKX. We were supposed to have a new generation of MK. We still may get it, but having so many similar characters smacks of the old "MK clone character" syndrome. Maybe not in execution (everyone may play differently), but certainly in character diversity and creativity.