Yes, it is a valid/useful diagnosis.

When you're depressed, your concentration sucks... but when you're in a better mood, your concentration is usually fine. (That's assuming you don't sit on the ADHD spectrum.)

When you have ADHD, your concentration sucks no matter what mood you're in (although depression certainly exacerbates those concentration issues). Not everyone with BP has concentration problems that exist 100% of the time (and I realize this is why you suggested the "specifier"). Also, there is much more to ADHD than attention & concentration issues. There is emotional dysregulation involved, and often hyperactivity with impulsivity exists. Sure, there are people with ADHD who don't have the hyperactive-impulsive behavior (and therefore qualify for the inattentive ADHD subtype), but their working memory and processing speed is impaired.

Bipolar disorder involves mood instability (hence it is a mood disorder), not attention/concentration problems. Sure, a mood can bring about attention/concentration problems, but if you don't sit on the ADHD spectrum, those problems don't exist when you're in a normal state of mind. I think ADHD is very much separate from BP because it is independent of mood and therefore the "ADHD symptoms" seen in BP patients should not qualify as a BP "specifier".

So, tl;dr -- I don't think it should be a specifier. I think the ADHD-BP combo is just a common comorbidity.

Same thing with BP people who have GAD. There are a lot of people here who also have GAD.