The more often you ride, then less your need for so much structure in your workouts. But if planning them and doing them motivate you then go right ahead. Mostly just putting in the mileage on varied terrain and picking a few places to really go at it will have you near the top of your abilities.
I've seen someone else mention it in another thread, but I have to agree with them that if you don't have much time to devote to cycling, but want to be able to ride at the level of others in a group, or just know your numbers are competitive with others, then structured workouts will give you a lot of bang for the limited amount of cycling time you might have.
If you can put in a lot of mileage per week, then you'll probably naturally encounter the things that workouts force you to do. Unless you are truly in the flat lands with no hills.