I don't know what YOU consider an "average" beer, but for any but the lowest gravities, starters make for better beer.
It's really a good idea to make starters when using ANY liguid yeast for all beers above 1.020 OG...
The biggest reason I suggest folks make a starter is if you make one
you'll have peace of mind.
And you won't be starting an "is my yeast dead" thread in a couple of days.
Making a starter first insures that your yeast is still alive and viable before you dump it in your beer. You will be less likely to start one of those "is my yeast dead?" threads that are on here every day.
You will also ensure that you have enough yeast usually the tubes and smack packs are a lot less yeast that you really should use for healthy fermentation.
Making a starter also usually means your beer will take off sooner, because the first thing that the little buggers do in the presence of wort (whether in a flask or in a fermenter) is have an orgy to reproduce enough cells to do the job...So it won't take such a long time in the fermenter since they started doing it in the flask.
Additionally it is better for the yeast to consume and reproduce incrementally rather than just dumping them into the fermenter...The yeast will be less stressed out than if you just dump them in.
Stressed out yeast can lead to a lot of off flavors...maybe even (though rare) the dreaded autolysis....Or the curse of 1.030....getting a stuck fermentation because the yeast have bit the dust.
So making a starter proves your yeast is still healthy, allows you to grow enough yeast to do the job, cuts down on lag time, and ensures that you will not get off flavors or stuck ferementations from stressed out yeast.
Also has to do with the actual pitch rates of the smack packs and tubes, and has to do with the data that Jamil Z has on his mr malty website.
I'll quote some of it, but really you should look at the stuff there;
http://www.mrmalty.com/pitching.php
But to make it easier he has a great pitch rate calculator
http://www.mrmalty.com/calc/calc.html
And according to his numbers on his calculator, really any beer above 1.020, you should be making a starter for.
Me personally when I use liquid yeast I just make a starter. I may not be as anal as some brewers and makes sure that I have the exact cellcount for whatever gravity beer I am making, but I do make one for the above reasons I mentioned, namely peace of mid, and a reduction in lag time.