Whether to wear a riding suit depends on the goal of riding.
For people who ride to enjoy a thrill protective riding gear considerably diminishes the sensation of danger, reducing the thrill and defeating the purpose of riding. People who ride to show how brave/fit/fabulous the are generally don't like to wear protective gear either, as it amounts to an admission of vulnerability thereby defeating the purpose of riding. A knowing passenger without protective gear offers a poignant expression of trust and submission to the driver.
The vast majority who ride "naked" simply hop on a bike and don't give the matter any thought at all, either out of ignorance or denial.
"I would rather die doing something I like than become a vegetable" is a common refrain from folks who dislike protective gear. Seemingly it's an argument of some merit. This writer tends to agree that it's better to be dead than a vegetable, but getting quickly and neatly dead is a fairly rare occurrence compared to getting slowly and painfully skinned.
In Great Britain in 1998 about 25,000 motorcycle crashes occurred. Only about 500 of those were fatal, without reference to neat nor quick, which surely some were neither. On the other hand about two-thirds of the crashes produced "minor injuries", which are the sort a riding suit helps immensely to minimize. Only about a quarter of the crashes produced "major injuries", which one surmises a suit can't prevent. For details click the link. From the numbers on the web page one can estimate a suit will come in handy about every thirty thousand miles for an average rider.
Riding suits are really quite cheap: A motorcycle typically costs $5-10k, and about 5-10% of that per year to insure. A serviceable set of riding gear costs about one year's insurance but lasts more than one year (hopefully!) and compensates against damage to something considerably more costly and unpleasant to repair than the motorcycle.
For riders of means who do not enjoy fear and care little what others think there are some practical advantages to wearing a full suit. Weather protection is nice if it's cool or rainy and allows dressing for the destination. The suit keeps road grime off of street clothes. A decent suit converts painful mistakes into merely embarrasing ones.
Ok, when the weather's warm yes, dammit, the suit's hot 8-) but the other advantages remain. In a sufficiently benign environment the pleasures of going naked are immense and justify the risks, as in other aspects of life!
From the stats located toward the end of this article riding gear is best added from the ground up: Boots are probably more important than gloves, and pants are more valuable than jackets. Helmet priority is left to the reader. Chaps are a separate topic.