Saves the developer's work I guess, not having to make sure a variety of ammo is available - I never really had a problem in DX unless I decided aiming was for fools. Making ammo need weight/inventory space or just fiddling with the availability (albeit a bit cheesier) is a bit better than having a sniper eat into his CQ weapon's ammo, for example.
Feel like playing a new adventure game, but not seeing one that catches my eye, so am going to replay Syberia.