& she spent 5 of the next 6 years trying to worm her way back on the show
This.
And they let her (!) return periodically in some of the most important moments, just in time to torpedo some of the most potentially great episodes by creating asinine new ways for her to appear just so she could be thrown a bone.
It worked just fine in "Yesterday's Enterprise" and "AGT," but RDM and others have since commented what a mistake the character of Sela ended up being, and they regretted creating it. It blows my mind they allowed that character to appear in
both "Unification" and "Redemption." It's not even so much Denise, it's that character.
She did indeed realize she made a mistake by leaving (and by the way she left....), but I wish it could have been handled differently.
because she's not that good of an actress, & the show was better without her
Agreed. In retrospect, actually one of my most important episodes of Season 1 is "Skin of Evil."
I know that's not popular, but I think it added a much needed dose of realism to TNG early on, and it ended up benefiting the series. Also, her holodeck memorial is a criminally underrated scene, and it's perhaps her best work on the show.
Most importantly, TNG did improve after the cast was trimmed and reorganized.
It was also rumored that she lobbied to appear in the Romulan plot-line in
Nemesis too, but they politely declined.
She does have her fans though. She helped the
Trekkies documentary (and maybe the next one too?) and there's a certain segment that really likes her. I'm glad for her for that, but they finally cut her off at some point because Shinzon and B4 were already quite enough in the bizarro-unbelievable-clone department.