I added a tank to my Iwata Smart Jet and am glad I did. The thing would run all the time you were using it just as it was supposed to but I found it annoying. With the tank it takes it about 95 seconds to fill on start up and then only runs about 20-25 seconds every minute or so while I use it. Pressure stays up because the tank pressure is around 50 and I regulate it down to about 12. Sometimes the thing never kicks on at all when I do small stuff like props and gear.
No matter what you do you will get a slight decrease in line pressure whenever you pull the trigger. Set your working pressure with the regulator while the trigger is depressed.
I'm a tank advocate and recommend it whenever possible thought it's definately not a requirement.
Edit to add: Just like Mike says, you need one that kicks on and off automatically like the Smart Jet does. By the way you describe it it sounds like your does too. I've never seen or used that particular compressor so have no idea.
About the only "gotcha" is you need to have a way to drain off accumulated moisture from the tank. Having a tank increases the volume of air and condensation is more likely.