Let me interrupt you there, if I may. The key word is "presumably". She has the experience--and therefore, skills--of eight lifetimes. And as stated, she's basically the communications officer on the Defiant.
She was seen doing somce comms work, which actually makes sense - Starfleet officer training is known to be well-rounded, so if she has enough expertise to function competently in that role, she might be assigned to it because of the general crew attrition issues caused by the war.
But, as I mentioned in my previous post, I don't think this sentence
She has the experience--and therefore, skills--of eight lifetimes.
is accurate if taken at face value. She cannot call upon them fully, at will. I don't think ANY joined Trill can, actually, not to the full extent that a person with actual training and experience could.
Let me see if I can illustrate how I view the passage of skills from one host to the next. Curzon Dax was a hell of a diplomat. This is because Curzon trained and gained experience
as a diplomat; becoming the next Dax host was irrelevant to that. Experience of prior hosts as diplomats/ambassadors/etc certainly would
help, and such experience would probably allow him to learn the trade more quickly, but he still had to devote time in his life to training and honing his skills to be effective in his chosen field.
Curzon's experience became part of the Dax symbiont's sumtotal experience, yes, but it doesn't just transfer cleanly and fully, like a file download. Jadzia would have some skills on that area due to the symbiont, but would she be as good a diplomat as, say, a Human who had undergone the same training and honing of skills in that field as Curzon had? No. Because Jadzia Dax never pursued diplomacy, never honed the skill herself.
Ezri gains even less in terms of direct skill transfer than most joined Trill would, given how unprepared she was. It certainly would help her, but simply having a skilled pilot as a prior host doesn't mean she'd be as skilled as a dedicated pilot, only that she would be more skilled than your average non-pilot would be (and if she wanted to BECOME a skilled pilot, she could no doubt pick it up relatively quickly due to the symbiont, but she'd still have to put in the work and time to hone her skills before being called "good" in the context of dedicated pilots). In the same vein, having Jadzia as a prior host doesn't make her a badass in melee combat (as we saw; she didn't fare well at all when forced to engage in it).
If a joined Trill actually gained - in full, to the same level of development as the previous host, and able to be called on 100% at will - ALL the skills of ALL of their hosts, then joined Trill with more than two or three previous lives would be far and away the most skilled officers in Starfleet. Jadzia would completely blow everyone else out of the water, by a WIDE margin, in every area that Starfleet trains in. This is not shown to be the case.
Well, I think the term "flake" is a bit unfair.
I wanted to touch on this myself, but I forgot: it's more than "a bit unfair", it's completely inaccurate. She was never a "flake" to begin with. A flake is someone who is unreliable, someone who cannot be counted on to follow through on what they say
because they end up blowing it off or not even trying. Ezri had legitimate troubles with some aspects of doing her job because of having a symbiont thrust upon her suddenly; this isn't the same as "being a flake."
On the subject of a counselor on the bridge--in times where the counselor is treating someone, it makes a great deal of sense that said counselor would be on the bridge/Ops.
Did we ever see this, though? With Ezri, I mean. Perhaps I am just forgetting something.
After all, the bridge/Ops crew are the central hub of the crew. They have the most responsibilities--with the possible exceptions of the chief engineer and doctor. As such, it is absolutely essential that they are in top psychological shape. Therefore, it is therefore ideal that someone be present at all times to observe the mental states of the bridge/Ops crew.
But she'd simply be called to the bridge when/if needed, like is often the case with the doctor. The bridge is a place of practical, ship-running work; you generally don't have people standing around without a very good reason. If Ezri is there because she is waiting for someone to need some kind of psychological help... that's silly if you ask me. If she is there because they have found an actual job for her to do, then that's fine, but in that case, she would have to
abandon said job in order to devote her attention to suddenly helping someone in need, and in that case, could only do so if someone else could jump in and take over whatever she had been doing (say, communications).
Now, as I said, it's not entirely unreasonable to assume that Starfleet's manpower issues would necessitate things like "Well, she's the counselor, but she's pretty good with communications, so give her that job since we're short on experienced personnel at multiple positions anyway." But I do think that's the case: in other circumstances, she might
not be on the bridge at all under normal operating conditions (especially on a ship like the
Defiant). And that's only the in-universe reason anyway: out of universe, I think it's exactly right that it was simply a matter of wanting the lead characters to have a presence within the "main set" of the ship, thus Ezri (and Bashir, at times) are on the bridge doing things, whether or not it makes sense internally (and again, it's part of a larger problem with DS9, that roles and jobs were really poorly defined).