I guess it is not so much Petrie's valuing Cera's opinon above other opinions, but rather that he tends to take the path of least resistance. Unless there is something of really crucial importance for him (e.g. LBT 7) Petrie will rarely take a personal stand on anything without being sure of support from the others. Sometimes I guess he would like to be more self confident in his actions but doesn't muster the energy to show it to anyone but himself (LBT 9: "They can't make me! Me think for myself!" "Petrie?" "Coming!").
In a way Petrie might be called (this is not meant to sound as harsh as it probably does) a "turncoat", always fluttering in the direction where the wind blows him. And if it comes to making points allowing for no opposing voices Cera is certainly the main producer of winds for the "turncoat" to flutter with. Ironically the image can be found in the movies too. At two times (I think there may be more cases) Cera is blowing Petrie away when he is opposing or bothering her.
