Episiotomy is a procedure as unnatural as any elective surgery. Most folks, to be sure, prefer not to be cut for any reason. But just because it's unnatural doesn't mean it's unjustified. It is actually a helpful procedure, along with the other unnatural aspects of labor and delivery—epidurals, IVs, etc. An episiotomy involves making a vertical incision in the tissue between the floor of the vagina and the rectum, thereby increasing the circumference of the exit for the baby's head at the time of delivery. It is painless when done under an epidural, local anesthetic, or pudendal (nerve) block. The circle of tissue that is the outlet for the baby is made larger and the chance of tearing is less.
Since it is easier to repair a surgical incision than a traumatic tear, the decision to cut an episiotomy is made at the last moment when it looks like there may be tearing without one. In this way, it should be looked at as preventative. But it only prevents superficial tears. No episiotomy—no increase in diameter of just the outlet—will prevent deep tears if the force of the delivery (due to a large head, forceps, or precipitous expulsion) exceeds the elasticity of the tissues of the pelvis. Something will have to give, and it's usually the baby's head that wins.
Some espouse never cutting an episiotomy. Although this philosophy won't cause deep tears, the superficial ones can increase the time of recovery by a few weeks. The gamble here, of course, is whether or not a patient will get away with nothing needing repair. It is tempting, but it is certain that a surgical repair of a straight incision hurts less and heals better than a disarray of tissue split in several different directions.
What's the best approach? Leave it up to your obstetrician—that's why you chose your doctor. Of course, discuss the issue ahead of time so that both of you are clear on a common plan. But keep in mind that your doctor doesn't do episiotomies if clearly not needed. They're included in the global fee, so there's no financial incentive, and it's more work to repair one than not repair one. So if it looks like the baby will deliver without unreasonable stretching and risk, your doctor will gladly skip the episiotomy. If it looks as if there may be some trauma, an episiotomy—the smallest necessary—will be used for your benefit.
Absorbable suture means you don't need the stitches removed. And because of the unique immunology of the area, infection is rare. The area ultimately heals well, even when there was tearing. In fact, it's often difficult to tell a woman's had one by exam. Thinning of the floor of the vagina is from the passage of the baby, not from the decision to do an episiotomy or not. One must remember that childbirth is an amazing phenomenon of physics, pushing a body's capacity to the max. Compared to the actual delivery, episiotomy can be thought of as an inconvenience or an advantage, depending on what could have happened with or without it. But it is a secondary consideration when an irresistible force—the baby—meets an immovable object—you.