Teams use more sensors not just because sensors are awesome, but because there is useful info to gather. This info can be used to make a more effective and robust robot.
If I were caught on a desert island and could only bring a few types of sensors with me, here's a few things I'd consider:
#1 - Encoders. Accurate measurement of wheel velocity and distance traveled on a drivetrain is crucial for a consistent autonomous routine. Also for consistency in wheeled shooters... Anywhere you would like something to be going a certain speed or distance, encoders are a better solution than hardcoding a motor control value.
One option:
http://www.andymark.com/product-p/am-2899.htm
#2 - Gyroscope. Generally, the more expensive the better. Gyro gives you rotational velocity of your robot. By integrating (accumulating) it over time, you can deduce the angle of your robot relative to the angle you started the match at. This is very useful for autonomous navigation.
http://www.analog.com/en/products/me...-evaluationkit
#3 - Lidar Lite. This nice little sensor measures distance with a laser. By pointing it out the front of a bot, you can figure out how far you are from a wall, tote, or anything else solid. Useful for autonomous, or for helping the drivers position the robot.
http://www.robotshop.com/en/lidar-li...FQgzaQodcHAIUQ
#4 - Pneumatic pressure analog sensor. Allows the drivers to see how much air they have in reserve. Not used all the time during competition, but helpful for diagnosing issues when you can't walk out to the robot to check on it.
http://www.andymark.com/product-p/am-3219.htm