Eh, there are other reasons to recommend the others. Kasumi is acknowleged by Shepard as being specifically good with unfamiliar technologies when recruited for the Crucible. Tali's not just from a technologically inclined society, she's considered a prodigy/genius within that society. Legion has been on the mission a short amount of time and has potentially had the least exposure to Collector systems (the other two potentially visited the cruiser, for example) or non geth technologies in general.
Environmental concerns though, yeah. Tali says she gets set on fire, which means that the suit probably saved her and Kasumi is likely not a very good choice there by extension. Legion is unquestionably best in that regard as long as the polymers and metals its platform is made of are heat resistant or shielded enough to not impair its functioning. Hell, jobs like that are the reason the geth were originally built.
I always assumed they wore the helmets/ masks on the cruiser because of the assumption that life support systems may be disabled, well before they realized that it was just TIM fuckery.
This is all academic though, because we know who is the best choice to go in the vents.

he did volunteer 
That may be all be true but it still doesn't make a difference in terms of who's better suited. Experience in foreign tech or being a prodigy still doesn't compare to synthetics hacking synthetics. Reaction times aside it's like picking a Westerner with a knack for languages over a native speaker to translate Russian, or Japanese. Both can do the job, but the latter is just the path of least resistance. And that's a poor comparison because a difference of languages doesn't even begin to compare to the difference in perspectives between organics and intelligent software that thinks and communicates at the speed of light. There may be unique exceptions (David from Overlord say) but overall an AI should logically always be superior to organics in the digital world. Unless the organics are digitized, or the AI is a neural net specifically limited and patterned on an organic brain.
And on the geth specifically, the reason they're so dangerous isn't just their platforms, it's because they can spread and hack through just about any system. Given their isolation, every case of this would count as "non-geth systems" and every time the distinction wouldn't matter.
But yeah, Jacob volunteered, so this is all moot.
You would've been correct if we talked about any human biotic but Jack is different. She is powerful not only due to conditioning at Teltin facility. One example of her uniqueness is the upgrade she gets in Mass Effect 2: Jack's vital signs show progressive neural degeneration, which would normally decrease her potential ability to generate mass effect fields. Instead, her ability appears to be increasing. Replacing the power module in her bio-amp with a higher-capacity prototype module would dramatically increase the strength of Jack's biotics.
Usually asari>humans biotically. In case of Jack, the question is in the air. We see her capable of the same shockwave as Samara on the Collector Base, see her floating like Samara (though less gracefully). I think she is capable to powers of the same strength but will burn out much quicker.
There is nothing known about environmental hazard in the vents. The only criterion is how good your tech expert will be. The job is basically why you would hire a thief. Tali is considered a tech genius in a tech-aligned society. More importantly, both potentially had encounters with Collectors. I don't know whether Legion will be able to hack Collector technology or not.
Do you have any evidence that said effects are explicitly natural? Because the Teltin experiments were quite extensive. The goal there was stronger biotic potential, not just stronger biotics. I'd be surprised if gene therapy wasn't among them. Point is, they tried a shitload of stuff on the other kids and anything that worked they then applied to Jack. In terms of her biotics I would consider her as "artificial" as Miranda.
I can agree she might be capable of comparable energy output to Samara, but like you said nowhere near as sustained. By most definitions of power I'm aware of, that does make Samara more powerful:)
I've replied to this above. The only thing I'll add is regarding the expectation of environmental hazards. Short version: of course they would be anticipated. First off, the precedent of the Collector ship is justification enough. Second, even if that was merely a precaution as QMR suggested... then it's a precaution that would certainly be repeated. And if you have the choice between a fairly exposed thief, a better insulated quarian, or a synthetic that just makes the whole concern a non-issue, well then isn't the no-brainer obvious.
PS Bonus points that it becomes a (possibly unintentional) homage to Aliens. Bishop in the vents, Legion in the vents... Both get rather large holes through their chests, though obviously it works out better for the latter:lol:
I don't think she's as powerful as Samara, but Samara is practically a matriarch. That doesn't mean Jack is on the level of average Asari. Some don't even develop their abilities much. Jack is a biotic specialist in ME2 for a reason. The whole point of the suicide mission is to gather the galaxy's biggest badasses. They are all meant to be larger than life. Like ME's version of comic book heroes. And now you're just relegating it to "average Asari".
Yes well, the average asari wasn't available at the time, and also wasn't expendable:P
On a more serious note, if you're going to bring up superheroes, please realize there are different tiers of power.

Case in point. A supersoldier, two spies, a guy in Power Armor, a god and the thing that smashes other, puny gods.
Edit: So do these forums shrink all images to dinky proportions now? Anyone know how to stop that? Specifying height and width attributes doesn't seem to do it.