If you have a 2+ hour LAX connection, the short-check of the bag to LAX is a good idea. It also may make your LAN check-in faster, as you still need to check in with them, and they will have to input all the data from your AS baggage claim check during your check-in. I've short-checked my bag to LAX several times. It is a fairly short walk for you and your luggage from T3 to TBIT check in.
The problem will arise if you have any sort of delays getting to LAX, i.e., missing the baggage cut-off time, etc. If AS accepts your bags all the way to MVD @ SEA, and anything goes wrong, your bags should eventually show up @ MVD. If your bags are late/lost getting to LAX, you are without luggage for your trip with a short-check.