Yes, you need one.
Smoking with the setup you describe has been shown to cause irritable bowel syndrome, itchiness, vertigo, dizziness, constipation, heart palpations, hypertension, hi blood pressure, impotence, incontinence, loose stool, hives, hair loss, dry mouth, blurred vision, insomnia, drowsiness...
Just ask the Surgeon General.
Seriously, as said before, you don't "... need" one, but they sure are fun and I'll bet you get better results. Also, the price point is not that high.
See my other posts but I finally made the jump last year have been very happy that I did. Almost too easy to use and a lot of flexibility to cook/smoke different things that won't fit on your current set-up.
After cooking with mine a few times, I threw a big BBQ party and invited over a ton of guys from the neighborhood. Late that Friday night, I put on 4 butts. They cooked all night. Party was at 2:00 on Saturday. More than one person said it was the best BBQ they had had in a long time.
To be fair, it was far from competition quality, but still very good. Now, how fun is that?
Good luck,
JKC