its not really about necessary but more of personal preference of both the crowd and performer.
The crowd might have two different opinions on it.
1." Just quit moving and play the instrument properly.The dude is moving like hell but he can't get hold note properly and his cables keeps on disconnecting".
2." Move lah!! so boring.NO LIFE"
While the from the band view varies from.
1."eh later,don't just stand there.Move and express yourself.Don't be boring"
2."I'll just play my instrument properly.I don;t want to look like a fool and screw up the song".
Theres many more reasons but then again this is all my opinion.haha.my 5 cents
Some local guitarist don;t move that much or rarely moves but are a good to watch and play damn well. Mark John of Allura and Field of caracal ( I think ) .
While some local musicians move and still do well at thier instruments.Caracal ( field,martin,gab,KC,Chris) , Josh of the fire fight, trella , Inch from allura ,Matt of AVA and Shah from Silhouette just to name a few.
( sorry if i got the names wrong or gave wrong opinions )