Outside of that mechanic, I am unaware of anything mechanical that an alliance allows that a corporation does not allow. I understand it does allow common standings between member corporations (which trickles down to members), but so can discussing standings list routinely.
Electus Matari currently has 788 standings set. There is rarely a day where we do not add or remove some standing. We have 2-3 people regularly set standings based on requests from about half a dozen diplomats, because just 1 person doing it tends to burn that person out pretty fast.
Of those standings, 170 are positive. Arranging for blue standings for a number of corps instead of just an alliance gets diplomatically much more complicated (BTDT, don't want to go back). And we already
have trouble with diplomat burnout.
So, no, I don't believe the shared standings are some kind of irrelevant detail that we could do just as well on corp basis. (We tried in the past, it's a nightmare.)
Another mechanical benefit are shared war decs. War dec cost from multiple corporations on a single alliance quickly become simply prohibitive.
I understand the non-mechanical perceived benefit of maintaining corporate identity, however to outsiders I think said identity is lost.
Yes, it is.
An alliance is a middle thing between a group of separate corporations and a single corporation.
Compared to the group of separate corporations, it gains much closer cooperation. As I mentioned elsewhere, when we dropped out of the alliance to join the militia for a bit over a month, we already noticed how we were "drifting apart" - we lost cohesion and a feeling of "belonging together", even though we shared standings and had an "alliance channel" for all corporations. The alliance really adds the feeling of "belonging together", and we do want that feeling. We are
proud of flying "as EM", and we
like others seeing us as "Electus Matari". That is, we like the shared identity.
Compared to a single corporation, though, alliances retain much more corporate identity. This is mostly important for our pilots. For example, BIONE and LUTI are totally different corporations (and both awesome) - the pilots there are different, the "corporate culture" is different, and what they do outside of alliance ops is different. Different people enjoy different cultures and "climates", so with different corps, there is much more variety on where to go and what to pick. We have had people move between corps in the alliance in the past, and that is fine - there is no "one size fits all", and people can join the corps they are comfortable the most, while still keeping in touch easily with others. For all of this, it's mostly irrelevant whether outsiders know what those differences are.
So yes, if your corporation does not want to give up some of its identity, an alliance will be bad for it. But an alliance offers different ways of cooperating than just merging corporations would (or trying for a "coalition of corporations"), and those ways can be useful for some people. So I wouldn't restrict alliance usefulness to sov-holding only.