If you have a portfolio filled with Photoshop, that's fine. Everyone else does. But don't expect people to be all that impressed. The last time anyone was impressed by Photoshop skills was 1995. And yes, you should have Photoshop stuff. But include Adobe Illustrator and you go to the next level. It shows that you understand a concept called "vectors".
As an employer, this has an additional advantage beyond Adobe Illustrator. Someone who knows vectors will be better at InDesign. And, unless you're hiring someone strictly to do web design, InDesign and Illustrator are the two things that a Graphic Designer needs to be the strongest at. Think page layout and logo design. Vectors.
|Adobe Illustrator - vectors|
Yeah, it's not fair that people who know Adobe Illustrator have an advantage. But who said that life, and the job market, was going to be fair?