So you know how when you're belaying you clip in to the belaying loop, and when your climbing you tie in to the two loops the belay loop is stiched through... When you're climbing, what part would you attach the rope to if it has a carabiner clipped to a pre-tied 8 in the rope ? (I know they usually do this when lots of people are in and they just want stuff to be faster)
Carabiners always attach to the belay loop. Attaching carabiners to the tie-in-points causes them to get loaded incorrectly.
Carabiners are designed to load the spine, which is the side opposite the gate. Attaching a carabiner to the tie-in-points causes the gate to be loaded, since three strands get loaded (the tie-in loops and the rope end). An incorrectly loaded carabiner fails at much lower forces.
A quick safety notice: When tying in this way, it is standard to use two locking carabiners. Never use just one non-locking carabiner!
If you are climbing toprope, then you connect the rope via locking carabiner to your belay loop. You do not have to expect high forces.
If you are leading you should tie the rope directly to the tie in loops. When falling in a lead you have to expect much higher forces than when toproping. They can relatively easily exceed the crossloading breaking force, and crossloading can easily happen as the rope is not under tension. If you absolutely want to use carabiners in this scenario, use two locking carabiners with opposing gates (even if both are crossloaded, they still share some load) or a carabiner that avoids crossloading with a bar, like this one:
You should not connect the carabiner to the tie in loops, as carabiners are usually not rated for ring loading.