I had the same debate but.....
Once you have bought the Hardware and Software plus consumables, unless you're selling loads of cars (or plates), I don't see a ROI. I pay £19.90 for front and rear with my branding and it takes 5 minutes.
The biggest issue though is all your "mates" saying "Can I just get a plate for XYZ please." Plus, I have already had one DVLA inspection, luckily, I am squeaky clean (same goes for HMRC, Insurance etc).