For the carb needle to come loose from the slide the retainer E-clip/plate has to come off of it. Put a carb together as usual with no needle E-clip and try to shake it out. Do it with the slide fully raised. I don''t think it can happen. The slide will hold it in OR not let it come up out the jet enough to get horizontal and exit the carb. If it did somehow end up on top the slide it would probably jam the slide from raising.
If it did somehow get horizontal and loose it would get sucked into a running engine.
If the needle is not attached to the slide the fuel will be metered too little or too much to the extent the engine probably would not run.
Too many if's.....
There must be a bit of confusion here as to what the needle is.
But...to answer the question...Yes, a needle is needed.