Squad leader spawn (SLS) is there to keep people together, rather then ahving everybody run off on their own phase and direction. The SLS increases the odds that people stick together and attack together, simply because they all spawn "together" (in the same place that is).
The system has it's flaws such as a lone SL hiding within a flag zone so that his team can capture the flag no matter how many times the enemy kills them. SO no, it ain't perfect. But if you looked around you may have noticed posts indicating that we will likely see spawns from vehicles such as a APCs in the future. Allowing the creator of a map to use APC spawns instead of squad spawns. But ultimately it will be up to the mapper (and testing) on what seems to create and encourage the most and bestteam play.