Many people who go out to the bar say that they have no intention of driving drunk. They don't think that they're going to be intoxicated, and they're just planning on having a few drinks with friends over a long period of time. Their body should have plenty of time to metabolize the alcohol, and they should be able to drive safely.
If this doesn't happen, they say that they'll just call for a ride or get an Uber. Their plan is just to make a judgment call at the end of the night, determine if they're too drunk to drive, and then take appropriate action. This all sounds very safe.
The problem is that your judgment is clouded by the alcohol, and you may have no idea how drunk you actually are. This could lead to you determining that it will be safe to drive when it actually is not. Studies have even looked into this and determined that people are wildly inaccurate when guessing their own intoxication level.
Moreover, since alcohol inhibits your ability to make sound judgments, you may do things that you know are unsafe or that you wouldn't do otherwise. When you're sober, you say that you'll call for a ride. When you're intoxicated, you may decide that you know you're over the legal limit, but you can probably make it home. This can lead to a drunk driving arrest, even though you had plans to avoid such an outcome.
If you do end up getting pulled over on drunk driving charges, the impact on the rest of your life can be extensive. You need to know about all of your defense options.