Re: Is a tsuba necessary?
As Chuck said, depends on what you're doing.
Have done extensive kenjutsu sparring with bokken, I would say it's a requirement if you don't want your fingers broken, grazed, cut or swollen.
In my kendo dojo you have to use them, even though they're not strictly necessary for what you're doing.
In aiki I've found it falls somewhere in between, but for form's sake I prefer them.
As for the leather ones, I've found the heavy plastic ones (or whatever they're made of) have always done the trick.