They don't stand out as individual sprites, they look bland and there's barely anything that can tell them apart as individuals. Improving the silhouettes by; Giving each sprite that something that makes them the "person" they are. Some might have really spikey hair, one a large cape, one some big weapon etc. Things that you can recognize even if it was only a silhouette.
I don't know if any of you watch One piece or have heard of it, but this is a great example:

I have no trouble telling these characters apart even though they are only black. Depends of course if you know them from the start, but they all give their own impression.
I wouldn't say silhouettes are extremely important, but more a way of seeing things and perhaps improve your sprites. A strong silhouette doesn't necessarily mean a great sprite, but it's a lot
better ground to start building on.