Yeah, but how does that relate with Spiderman and his adventures? Just being a seamstress and being with Peter for a job doesn't mean MJ gets to participate in the bulk of the book, that is, Peter being Spider-man, that is what we are here for.
And of course, the same could be said about aunt May, or any of the supporting cast, but there was no stuggle to write aunt May because she ain't that important to Peter's story. Yes, she is Peter's mother for intents and purposes, and she does provide some purpose and drama for the story, like her being in her death bead or having bills to pay, but you could go on with a book for months without her showing up because not only Peter is already a grown ass man, keeping his Spider-man life from her also means she doesn't have a stake on whatever Spider-man does.
MJ... ain't so simple. She worked well at the start because she was the third piece of the triangle that pushed Spiderman to be a hit with teenagers in the 60's, turns out people like the Archie, Betty and Veronica dynamic and it was clever of Lee and Romita to use it, and turns out that if you use soap opera elements on you super-hero comics, they tend to work well, Claremont would be doing the same later on with the X-men.
And so the romance and such worked well because it was the juggle between Peter and his Spiderman life and how he managed them both, you know the drama stuff, and for this purpose, it was pretty clear her role in the story, to be one of Peter's love interest, to see if she and Peter will be togther, and that is the exciting part of it, that is why the heroes usually "get" the girl by the very end of the story.
Then Gwen died and Peter married MJ and now what? She knows Peter is Spiderman and aside from being the dame in distress in some stories or helping out in some tangential way in others, you could tell that they were struggling to find something for MJ to do, and you can't just write her as Aunt May, someone Peter occasionaly visits from time to time, she is a major part of Peter's life but not really Spiderman's.
Like most of the fans, I love MJ, and I couldn't help to smile when she was back in Spenser's run, but, like I said, I kinda unserstand why writers don't want her around, it is easier to write Spiderman going on this long crazy adventures, or doing the stupid shit he did on the Slott's run, or joining the Fantastic Four and shit if Peter doesn't have a wife to return to.
What can I say, Spiderman is 60 goddamned year old, and he never got a break from a ongoing story (with multiple books and events as well), there is no earthly way to keep a story that continues building on lore and drama going for this long without things going out of rail.
In a ideal world, Peter Parker's story should have ended when he married MJ, yes he would still be Spiderman and going to fight crime and stuff, but that we would not have to see, the send off would be that we get to see the hero get the girl he loves and be happy ever after, and we don't need to see said after, because it ain't interesting.