It's hardly about the worthiness of the Inquisitor. The Ben-Hassrath offered the alliance out of convenience, and it seems, according to what Sten and Iron Bull have said in the games, that the Ben-Hassrath doesn't really seem to be an organization that would think twice about turning on their word if it furthered the spread of the Qun. It seems more like it comes to a choice can you trust the Ben-Hassrath. Can you trust that once the crisis with the Rift isn't over, they wont see that as an opportunity to spread the Qun by force, and even worse use their alliance with the inquisition as a ploy to do so? Using their alliance with the herald of Andraste to make people think the Qun is what the maker wishes. Why else would the Herald have formed an alliance with the Qunari if their maker frowned on the Qun? And as Iron Bull alluded to, the powers that be in the Qunari society aren't above spin doctoring to make their stance seem more powerful. (such as claiming to have ancestry of dragons to make them seem more fierce)
Bottom line the Ben-Hassrath are untrustworthy, and they aren't worthy of an alliance with the inquisition.
Of course what the Qunari offer is an alliance of convenience, with no promises whatsoever about what they'll do once the threat you ally to deal with is over with and with no qualms about betraying their word if doing so would further their political goals more than the short- and long-term harm done to their diplomatic reputation from doing so would.
That's what alliances are about, be they alliances of convenience or necessesity. There are no guarantees about the future - only current common goals to be addressed.
Diplomats know that. Alliances are temporary agreements between parties made to achieve common goals, and the worth of the alliance is measured by what the parties do to complete the goals and how it affects the situation that it is supposed to address.
If the other party is friendly, that's a bonus, but it is certainly not a requirement. Allying with opponents and even traditional enemies to achieve common goals is entirely normal. (Allying with current enemies is much less frequent, but even that can happen when both sides face greater threats than the other)
Of course the Qunari will try to use any influence or opportunities they acquire from the alliance to advance their own agendas, just like the Inquisition will - at least with a competent leadership in charge - and this is something you take into account when you decide whether forming an alliance is worth it in the first place.
And if, while the alliance holds, you pursue the goals of the alliance, are willing to make sacrifices to achieve the goals, and follow the plans that are agreed on, this is something that other parties will note, as it shows you are a reliable ally that is worth having, while if you do not pursue the goals or are unwilling to make sacrifices to accomplish them, it shows you as an unreliable ally - one that is worth little, as you cannot be trusted to pursue the common goals that were the primary purpose of the alliance in the first place.
And doing what the poster I originally responded to said: "I don't mind giving any kind of alliance, temporary or no, some kind of chance. I just won't sacrifice anything or anyone on my end to get it.", that is an approach that will soon show everybody that you are worth little as an ally, as you are in it for the benefits and unwilling to pay the costs, thus poisoning the diplomatic waters with everybody.
A worthy ally is one you can count on to do his part, be it small or be it big, in achieving your common goal. Somebody whom you cannot count on to do his is unreliable, and if the reason you cannot count on it is because he deliberately sabotages common plans to avoid risk or shift it to alliance partners, he is unworthy. (Which is the example of bone-headed diplomacy I was answering to in my post.)