Someone who is a) under pressure from the project's irreplaceable head scientist and b) frankly in need of an expendable test subject, given what happened to the last one.
Especially given the highly likely chance that the test subject will die in the experiment, someone who's got nothing to lose is a pretty good choice.
In one episode (where the gland was temporarily non-functional on top of everything,) Darien tells Eberts that this actually made him an attractive test subject aside from his irreplaceable brother's insistence; his skills let him hit the ground running when he starts as an espionage agent.
The Quicksilver residue. Every time Darien goes visible, it flakes off in piles of what Claire dubs "$6 million dandruff" and takes 2-3 days to biodegrade, making it pretty clear where he's been if one knows what to look for. Taken to an extreme near the end of Season 2, when Darien begins becoming resistant to basic Counteragent and Claire tests a new formula; not only does Darien go Stage 5 within hours but the flakes he leaves behind are so concentrated they send other people into Stage 5 just by touching them.
There's something chilling about the idea of a sibling using you to test a risky gland. Plus that trick Kevin pulls with the spiders is pretty unpleasant because as Darien's brother he would be well-placed to know and exploit Darien's fears.
So the horror of having to kill a 12 year old (who is carrying a deadly virus) to save countless others is explored in the episode "Johnny Apocalypse" but it is also horrifying that Alex used Darien and Bobby to help gain Adam's trust and take him away to the secret lab without telling them what they were really doing or why.