If it had infinite lifetime, it would crash the game if you aimed it to the edge/sky of slate.
Er, no. That's with raycasts.
The problem with infinite projectiles is that there is no clean up. ever. If your projectile is cast out into the void it remains on your RAM and is instantiated on all client's computers (and all new clients that connect) far, FAR after it serves any purpose.
Setting the projectile to a very long life (even if it's like 3 minutes long) is still infinitely better than infinite life.