-Wavedash out of shield is ALWAYS faster than simply dropping shield and gets the extra utility of being able to move more quickly than simply dropping shield.
-Wavedash allows you to get onto the stage in the fastest manner possible from the ledge and allows you to maintain invincibility while doing it if you are fast enough.
-If you are trying to get to a platform above to use a ground based move, then jumping and airdodge down is the fastest way to land on it. Or, it allows you to move across the platform very quickly (wavedash)
-If you dash too far in one direction and need to stop, then letting marth go through that animation where he skids to a halt is very slow. You can cancel it by jumping, then go into your wavedash to quickly change direction or go straight into moves other than JC grab or crouch Dtilt.
-Wavedash from standing is the quickest way to reach peak walking speed (I believe)
-Wavedash is probably the quickest way for Marth to steal the ledge from people. You walk towards the ledge, dash away, then wavedash back to slide off the stage facing the correct direction. Finally, once off the stage from the wavedash you fast fall. Alternatively, you could SH Bair, then fall onto the ledge, but this is much slower than what I described before.
Those are just about the only uses I can see for wavedashing.