Your question appears to indicate a general uneasiness with the use of degree titles, even in contexts where expert knowledge in that field is relevant. There may certainly be some contexts where use of the academic title of "Dr" constitutes "flaunting" your degree, and is pretentious. (If you're on a plane and someone has a heart-attack, and the flight attendants yell, "Is anyone here a doctor?" then I wouldn't recommend putting your hand up.) However, in academic and professional contexts where your skills and training are directly relevant to your authority in that field, it is not unreasonable to use the title that signifies your training. Few would regard this as "flaunting" your degree, and for those that do, it is likely to be based on a more general objection to titles in general.
It is worth noting that in an academic context, the title of "Dr" is not really that high an attainment, relative to other staff. (Titles of "Prof", etc., are generally more impressive.) As you point out, most academics have a doctoral degree, and that is gradually becoming the minimum expected training for an academic. Thus, you are totally correct when you say that this doesn't get you very far --- it is a baseline level for the vast majority of entry-level academics. If you use your title then that is fine, and if you don't use it, people will probably play the odds and assume that you probably have your doctorate.
Personally, I think it is legitimate to use your title in any academic or professional context (i.e., papers, grant applications, correspondence, email signatures, etc.). In such contexts, your education in your field is potentially relevant, and it is unlikely that anyone would hold your use of your academic title against you. Moreover, people do not expect you to have to change your email signature to remove titles in contexts that are less formal. When you have back-and-forth email conversations with people, your signature block will only appear in the first email, and after this you can use informal sign-offs, so it is unlikely you are going to look pretentious.
All senior people in my field have doctoral degrees, so it doesn't really signify anything.
Kinda. But then, by implication, the absence of a doctoral degree would be unusual, and would signify something potentially significant. Use of your title of "Dr" indicates that you have the academic training that is standard for that field. Possession of a doctoral degree might also be required for some tasks in your field (e.g., supervision of doctoral candidates) and so it is legitimate to signify that you have this degree.
...often when people use their title it seems like they do it to show they are an authority of sorts...
And that is illegitimate how? Possession of a doctoral degree in a field (or a medical degree for MDs) is a legitimate indicator of expert training in that field, and therefore valid information suggesting that one is indeed an authority in that field. Having a doctoral degree in your subject puts you at a level of knowledge that is far higher than the average person and so you are indeed an "authority of sorts". You needn't shy away from the fact that you are highly trained in your field, and neither does Dr Phil.
