Your body wants it.....until you start going up a hill.
You feel weak and lightheaded....until other riders start dropping away from you on a hill.
+1 on that one.
I tend to do this AFTER my longer group rides mind you. Have you had a bacon cheeseburger and a brew after a ride? It's heaven on a plate and zen in a bottle. The ride makes it that much tastier.
Think of it differently. If you want to lose 2 kg's it will take about 4 weeks(500 grams a week). That's 4 weeks where you will limit yourself from "stupid" foods like that bacon cheese burger and a brew. After 4 weeks you've reached your goal and then you can start treating yourself to a bacon cheese burger and a brew, because at the moment you probably maintain your weight with your current diet, so no worries.
So ask yourself - is a 4 week diet worth dropping 2 kg's for, maybe losing that last fat around the belly? I love to have a good steak and a beer after a 3-4 hour ride with good intensity, but I'm willing to make some sacrifices if it means that I will be lean when the season starts.
Also, there's nothing wrong with treating yourself to something "unhealthy" once in a while, even when you're on a diet. Just remember to keep it to a minimum
And yes, keeping close to 500 kcals under what you should eat is tough, but I've not seen any decrease in performance. The secret is to get a good portion of carbs just after the workout to load the muscles, and maybe eat a banana just before the session.