Damien Sandow doesn't have a streak because he hasn't competed in a match yet, he attacked Yoshi Tatsu last week and that was his first physical altercation since the vignette's began playing hyping his debut. Sin Cara went undefeated for two months but lost his first match to Christian, he then proceeded to lose another couple times after that, one I remember being to Mark Henry.
Technically, Brodus Clay isn't undefeated, he is since his re-emergence but pre-Funkasuarus he lost a handful of times on both NXT and as the bodyguard to Alberto Del Rio. Cesaro has only had two matches, hardly a dominant winning/undefeated streak to brag about.
So really, Ryback and Tensai, due to them being totally repackaged are the only characters left with undefeated/winning streaks. They're both being built as monster's so it is only natural that they'd act like monster's, monster's don't lose, they destroy and win emphatically. Tensai is gaining small pockets of heat and although he is the subject of ignorant, smarky "Goldberg" chants, Ryback brings more entertainment to weekly television by annihilating a jobber than most current Superstar's can do cutting a promo.
In their cases, yes, they need the streaks to cement their characters as being dominant forces, so that when they eventually lose, whether it be to an established Superstar or another debuting Superstar, or even to one another, It'll be received as a huge win for said person and will gain them some recognition as being the person who ended said person't undefeated streak.