I'm no Soaps expert but I am pretty sure that that's how it goes on all Soap Operas. But I agree, that Jessica was worthless and perhaps the dumbest person in the world.
As discostu2214 stated, it pretty much has been a standard procedure in soap operas to recast characters. As to why they recasted characters that may have not been as important as others, I think JER may have been attached to the characters. In the early years, Jessica really didn't have much storyline of her own but was a champion of "Charguel" and tried to thwart Kay's constant manipulations and schemes. Then, later on, we had this good-girl-gone-bad storyline as Jessica got caught into a troubled world of prostitution. So, yeah, Jessica may not have been the most vital of characters but I feel like JER had plans for the character. Eventually, the story ran thin and I think he finally realized that it was time to write Jessica out. Same goes with many other characters. The only real recasting mistake I feel was made was the recasting of Charity. That was a complete and utter failure! The actress that they cast was a good actress but I don't think (at least in my own opinion) that she could fill the shoes of Molly Stanton because that's who Charity is! Even the casting department and JER must've caught on because that storyline was scrapped quickly. Some recasts work, others don't.
I swear when Jessica the prostitute came on, I was confused as to when Kay got a sister because I didn't even remember her being there on the beginning. That's how useless her character was. I'm watching from the beginning now and I had no idea Mary Elizabeth Winstead was the original. Good actress, probably the best of the younger set, but her character is SO annoying and so mean to her own sister. Yes, I get that Kay is the villain and trust me, I hate her, too, but Jessica is just over the top and mean. Also, they write both Kay and Jessica as younger than they are. They run around the house chasing each other and fighting like they're 10 and 11. Not 15 and 16. I can't stand either of them, really.
When Blair Redford, the last Miguel came on, he made Miguel my favorite character. He was by far the best. Serious as hell. Better than the first and second Miguel. He became my second favorite actor aftercPassions and I've seen him a lot since. I love how they made Miguel so serious compared to how he was earlier.
I love you, Kristen Stewart. :) You are so beautiful and talented. I would love to perform with you.