Good defense saves pitchers and allows for you to prevent hits. It's really a balancing act as good defense tends to cost more than poor defense. But first you have to understand how WIS does defense. Defense is broken out into 2 components on the site: Fielding and Range.
Fielding is always the simplest for me to understand, as it is essentially fielding percentage - or roughly the number of errors that you can expect your guy to make. Since errors are a known stat, you can easily see the difference of an A rated fielder with few errors and a high fielding percentage as compared to a D rated fielder with many errors and a lower fielding percentage.
Range is a whole different animal. WIS used a relative range factor system to determine range. This relative range factor determines whether or not the fielder will reach the ball. NOTE: Speed or SB numbers DO NOT factor into this...it is solely the range of the fielder.
The way it works is when a ball is put into play, WIS determines where it is hit...as an example, lets say it is a fly ball to CF. WIS then uses the Relative Range Factor of the player to determine if that player can reach the ball to make an out. If you CF has a high relative range factor then he will get to more balls than a CF with a lower RRF. Each player's RRF can be found by in the player search by choosing the RRF - OF stat category - FYI the letter grades give you an estimate, but not all A+ (or D- for that matter) are created equally). Once the sim determines if he can get to the ball or not, his fielding percentage comes into play to see if an error is made or not.
In my example, let's say your CF is 1984 Kirby Puckett. His RRF is listed as 3.49, one of the highest on the site for OF. Kirby will get to more balls and thus turn more would be hits into outs (+ plays in the game). If your CF were 1992 Rickey Henderson, he would make far fewer + plays as his RRF is only 2.18 even though his speed is higher than Kirby's. That makes sense you say because Kirby has an A+ range and Rickey has a C+ range and you are correct, but note that all letter grades are not equal, as 1980 Rickey Henderson is rates A+, but his RRF is only 2.87, so by the way WIS does defense, 1980 Rickey will not be as good defensively in CF as 1984 Kirby.
The other aspect of defense is that WIS normalizes it to the era of the pitcher on the mound. So pitchers from older eras where the fielding percentages are lower will make your whole team play worse defensively compared to pitchers of the modern era where fielding percentages are higher. I'll admit I'm not 100% sure how this works, but is another thing to keep in mind.
There is much debate as to truly how important defense is. Some swear all the + plays and the lack of - plays helps you to be able to draft less good pitchers or fewer IP. Some say it doesn't matter how bad of a defender a player is when he has a 1.15 OPS and hits 45 HRs. It is a big debate. My advice is to determine which strategy you want to use. Also, I would try to avoid any D- fielding or range, as these players tend to perform very poorly defensively in the sim. Also, playing players out of position tends to be a terrible idea as well as WIS uses a defensive spectrum to determine just how bad a player would be playing out of position.
Hope it helps...and good luck.