Yup, a spool is essentially a locked front axle.
Compared to a conventional diff, it doesn't turn in as well because of the lack of differential action. Thus, it can be said to be more stable.
The locked action allows maximum on-power steering. You'll find that under hard cornering on asphalt, a car with a front diff will pull towards the outside on-power. A one-way or spool car doesn't have this effect because of the locked axle on-power.
I've never raced on carpet, but I've heard that spools aren't really ideal for carpet because the high amount of traction available on carpet.