I've seen this topic discussed many times so I wanna weigh in. Might be a bit long winded.

The issue of Jimmy Steve is one that doesn't sit well with a lot of people mainly because they binge watch the show/have a repeat viewing and everything that happens with the character sits fresh in their minds and directly clashes with what later seasons try to say about the whole situation. What the show does is, instead of keeping continuity with actions of the characters, they kind of gloss over big events for the sake of keeping the action moving at a brisk pace and to stop characters from stagnating. So for someone who watched the show in one view years ago and never rewatched, or is just a sporadic watcher, it's easy for the writers to sell a certain narrative. In the case of Jimmy Steve the narrative of the newer seasons is "psycho" or "asshole" when in reality, if you actually have a good concept of what went on for 3 full seasons of character development, you'd know he was a far better character than what the show lets on and was most likely slated for far more before the writers started to really branch out for the foreseeable future. Let's break it down a bit so I can explain myself a little better.

Jimmy Steve was introduced, and this is backed up in interviews, as the Romeo to Fiona's Juliet in a fucked up love story. They were, contrary to what the show nowadays might have you think, meant for each other canonically based on the UK version. But how boring (and short lived) would the show we all love be if the main heroin found the perfect love interest in season 1-3 and lived happily ever after? The character of Fiona would stagnate, as she would have reached her "main" plot end, and the show would be significantly shorter. This is mainly why so many character spin their wheels, getting into similar situations, having several partners (just look at Mickey and Ian). If Mickey wound up with Ian, there go further seasons of tension, even if they are "right" for each other and ultimately will end up together much to no ones surprise, the writers want to drag this out as long as possible and create false drama.

Much like Mickey, Jimmy sheds his asshole persona gradually and becomes a real character. He jumps to save Monica from dying, he jumps to stop Grammy Gallagher from shooting Shiela and saves her, forgives Ian for the whole dad thing and it's clear he loved Fiona and the kids because he puts up with them treating him like shit for 3 seasons. And yes he lies and steals but he is nowhere near as morally reprehensible as even the youngest of the Gallaghers, and herein lies the problem:

When "Jack" comes back into town, it's as if the writers took all that growth, all those serious and heartfelt moments, and in one big act of glossing over they threw it all away to give Fiona a moment. He went from main character to plot point, and it showed. So she sees this person she loved, and after careful consideration sends him off even with all of the history. She was finally "free" of the bad boy, able to choose anyone she wants...and she ditches her other plot device husband and knowingly chooses a drug addict after almost killing her brother with cocaine a season earlier. So all of it (Jimmy leaving in the first place, Jimmy coming back in season 4, Gus, Sean, Seans problem etc) ignores logic, the shows continuity, and seasons of character growth because ANYTHING is more dramatic than Fiona choosing the person she was supposed to wind up with from episode one. And to top it all off, the writers jam it down further with the "he's an asshole comment", because yeah at this point he is an asshole, tarnished as a character for more seasons. Franks liver is another example, it was just too early for it to actually mean anything to the story. Frank without drinking means his end, just like Fiona with Jimmy means her end, and Ian with Mickey means his end. If he ever does come back, they would need to save face by saying something like he WAS telling the truth about the Dubai trip and just told that girl to say those things to Fiona to help her move on. Or maybe they wont. At this point, the writers can do literally whatever they want.