i've been fortunate enough to have a lot of admin positions wherein the role was respected. what i've personally found is that within the org, it's generally obvious to be respectful of the admins (we know virtually everything and everyone, we've made the right connections, we can whisper in ears to make things happen for you - both good AND bad, career-wise or even just getting timely help from IT - and we can kibosh [or make possible] your opportunity to work from the company by merely saying, "i didn't get a good vibe from that one" or the inverse); where i experience the most disrespect is from outsiders when answering the question, 'what do you do?' "i'm an executive assistant" is often answered with, "oh, so what do you WANT to do?"
i will say that i've definitely had thankless roles, though, like the one you describe, but it was usually a matter of the boss him/herself being a dick in general, and these were companies in which NO ONE was happy because NO ONE was being treated properly. otherwise, i've mostly had good experience in this arena when it comes to not feeling like i was performing a thankless, unimportant role. the jobs i've had where i did feel that way, i bounced at the earliest opportunity, and there were a few.
in my current company, i've outlasted seven bosses, am well-compensated, and am treated respectfully by everyone [and awkwardly deferentially by some].
that said, i'm also last on the list for spot bonuses and other financial incentives that are handed out like halloween candy to everyone else, sometimes to the same people repeatedly despite basically doing exactly what they're already paid well into double six figures to do (and due to my role, i see these particulars).
when i do my job expertly [which i do, every single day, and better than all of my counterparts - which they will admit and which management definitely recognizes)], it's 'meeting expectations,' even while out of the other side of their mouths, they'll mention all the ways in which my work product is superior to that of my counterparts. (isn't that inherently "exceeding expectations"? apparently not. worse yet, i'm close enough to my counterparts to know that they routinely get "exceeding" on THEIR reviews, and they end up outraged on my behalf.) when i go above and beyond for someone, whether a colleague, someone i don't support (including low level employees who should absolutely NOT be co-opting me as their by-proxy admin), a client, or a vendor, it's just 'doing my job.' when i arrange an office and bring fresh flowers and lunch and snacks and cold waters in for a candidate who will be there interviewing all day, it goes unnoticed, even though it's a step i take that i'm not asked to take (because no one else even thought to actually feed and refresh someone there for eight hours in a series of exhausting red robins) and which others have now started trying to emulate and demanding of their own assistants.
i actually support four people, three of whom DEFINITELY recognize my contributions and would like to reward me for them. unfortunately, my big bad is in the C-suite [who is the current object of my derision], she makes the calls, and her calls are generally 'meh, you're just doing your job.' she also doesn't ask the others i support, or even my counterparts, for their insights/experiences/opinions. while she likes me and the job i do and treats me respectfully, we definitely don't have a 'right hand' sort of relationship, and she doesn't weigh support roles heavily. or at all.
she's actually in the process of clawing back a part of my retention plan offered to me two years ago when i tried to resign (for a MUCH better position), because she has apparently decided my bonus last year should now reduce my salary.
now i'm just venting about my personal beef at the moment, but the point is:
admin work can be great and rewarding, if you work for people who appreciate you and WITH people who respect you. if you don't, it's the wrong gig. and unfortunately, i do currently find myself in what now may very well be the wrong gig. i've had good luck with execs in this company for seven years, but my current one... she's a different story. i don't even think she would've retained me if one of my other execs and the CEO hadn't insisted upon it when i turned in my resignation.
btw, as a note for anyone trying to turn an admin position into something more: i've seen this happen, but very very rarely. when people hire admins, they're looking for support, not for the next up and coming director/VP. in fact, i've interviewed for several positions wherein i've been asked if i wanted to do something more eventually. the times [before i decided to stick with admin work] that i answered honestly, i did not get called back. i've even had recruiters specifically instruct me to "not mention wanting to move up, even if you do, and by the way, maybe wear a string of pearls" [i didn't take the latter advice, though i took the former and indeed, got the job].
so my advice to people trying to move up the ladder from admin to something else is to be honest about your ultimate goals; you may lose out on the gig, but at least once you land one, you will be looked at with a different eye than "that's just the secretary." i'd also recommend, if you have a job wherein your ultimate goals are acknowledged, that you work with management on a plan to move up, and take the steps they recommend to achieve it.