Simple answer has to be No.
Wizards are not "essential" in groups. No class is actually "essential" since you can have groups without any given class as long as the function of that class is covered in some other way.
Wizards are useful because they can deliver significant damage and do it very quickly. They also have port and evac capabilities. Any group looking for a damage dealer should seriously consider a wizard.
The trouble is that a badly played wizard is very literally a loose cannon. One who nukes too early or cannot control the aggro generated by their nukes can slow down or endanger a group. This can make people wary of wizards they don't know. Of course once you know they are a nuke-happy blastaholic you're happy to have them along
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Wizards are also well capable of soloing if they want to or if there are no groups going.
As far as tips are concerned. Learn to control aggro, don't nuke too early and don't nuke and immediately sit unless the tank has very firm aggro.