Is establishing yourself as a reliable ally an instrumental or terminal goal for you?

Instrumental.

If the former, what advantages does it bring in a group blog / discussion forum like this one?

Trust, mostly. Which is itself an instrumental goal, of course, but the set of advantages that being trusted provides in a discussion is so ramified I don't know how I could begin to itemize it.
To pick one that came up recently, though, here's a discussion of one of the advantages of trust in a forum like this one, related to trolley problems and similar hypotheticals.
Another one that comes up far more often is other people's willingness to assume, when I say things that have both a sensible and a nonsensical interpretation, that I mean the former.

The kind of alliance you've mentioned so far are temporary ones formed implicitly by engaging someone in discussion, but people will discuss things with you if they think your comments are interesting, with virtually no consideration for how reliable you are as an ally.

Yes, I agree that when people form implicit alliances by (for example) engaging someone in discussion, they typically give virtually no explicit consideration for how reliable I am as an ally.

If you mean to say further that it doesn't affect them at all, I mostly disagree, but I suspect that at this point it might be useful to Taboo "ally."

People's estimation of how reliable I am as a person to engage in discussion with, for example, certainly does influence their willingness to engage me in discussion. And vice-versa. There are plenty of people I mostly don't engage in discussion, because I no longer trust that they will engage reliably.

Are you hoping to establish other kinds of alliances here?

Not that I can think of, but honestly this question bewilders me, so it's possible that you're asking about something I'm not even considering. What kind of alliances do you have in mind?