I assumed I wouldn't be so hot on the grenade launcher, so I didn't use it. But I was wrong, soooo wrong.
First, after the first few rounds, any civilians near the aliens are usually dead, especially if an alien is camping in a small building. Second, there are places where aliens hide and civilians don't, such as in their ships.
Ah, the grenade launcher.
It can actually shoot farther than the firing arc allows, if you use the "timed" mode. Fire it so it bounces along a little farther. Seriously, I've bounced grenades through windows into little buildings and killed aliens inside. And bouncing 3-round bursts of grenades around inside a harvester or supply ship is just awesome. Does all kinds of damage. The grenade launcher is a highly specialized weapon, and must be used with care. It's not the weapon an agent should carry storming a room, or working closely around civilians. It's a fire-support weapon and, when in its element, does awesome damage. It's great for dislodging ensconced aliens. You can bounce timed grenades around corners, too.
I don't fully understand the flechette rounds, which seem to fire like a monstrous shotgun with a wide spread. In this case, the flechette rounds actually make a great storming weapon, because nothing, and I mean nothing, at close range can survive one shot of it. I thought the flechette round would fire like a grenade, then explode at the point of impact. Either way, I equip my Grenadier with a sidearm so he's got something suitable for close-quarters fighting, and I'm thinking to upgrade him to carrying an SMG for that purpose.
So, yeah, the grenade launcher is awesome. Try it out!