I have some tea infusers, and rarely use them. You need big ones. 3-4 inches for typical additions of up to an ounce of hops. They do cut down a bit on the debris doing into the fermenter, but lots of other techniques do too, I usually use a strainer which also removes debris AND helps aerate.
I use the tea balls when I really don't want hop debris, say when I'm planning on re-pitching the yeast. I recently made a 3-gallon patersbier with the intention of reusing the yeast in a 5-gallon tripel. There were only 2 small additions of only 0.25oz each, and I also strained the wort. I did not whirlpool. When I finally transferred the patersbier into the keg as I was brewing my tripel, the remaining cake was beautiful. It just looked like a giant vial of yeast, no hop debris that I could see.
For most brews, I think a whirlpool or a strainer does well enough, especially when coupled with good racking technique, to keep hops out of the bottles or keg.
Do you need them? No, but I f you want them, get them.