An Equestrian enters the portal

@Twi_Clown
Because it shows off how special Starlight is, particularly to the writing staff. No other characters sans Princess Twilight has managed to go to EQG but now Starlight can as well, which just pushes home unique she is.
Then there’s the fact that EQG is the black sheep to many, that the stuff in the show will get pushed into it while EQG remains constantly blocked off from the show. Starlight can appear in both the show and EQG, where as Sunset can ONLY appear in EQG.
And then that leads into the Sunset Starlight skuffle because they share similar backgrounds and functions, it is the show stating Starlight as more legitimate or the one the writers push more. So putting in Starlight basically pushes out a lot of the unique things EQG has going for it in favor of something the show already has and is a contested pary of said show.

@EotD
^THIS.

Having Starlight in EG again is sheer redundancy.