Getting a flu shot is a little like flood insurance. If you are frail, have a compromised immune system, or if your health is brittle, then the slightest flu can really be a health risk. And we go through every day exposed to such risks. And then as JeanV mentions, there is a flu of epic proportions. I am reading (actually listening to it) a book right now in the 1918 flu "The Great Influenza." And it took out young and old. This is the 100 year flood. In my view, protecting the nation against an epidemic would require forced injection of everyone, not something we are likely to see.
The recommendations that everyone with diabetes get a flu shot is based on the idea that everyone with diabetes is in some way health compromised. If you have ever had the flu and been really incapacitated, ever had trouble with pneumonia, have any chronic organ disease, ever ended up in DKA form what seemed like some little cold, then you are at increased risk. But we have to understand, the majority of deaths from flu are in those over 65 who have much greater risk. In general, a person over 65 is 100 times more likely to die from influenza than someone < 50.
Vaccines tend to either be dead or attenuated. In the case of the flu, the shot is a dead virus and the spray may either be dead or attenuated. The vaccine works by generating an immune response, preparing and strenghtening the body against any future encounter with the virus. You should have some reaction, but in most cases it is mild and at worst a slight inconvenience. You should not get the flu.
In my case, I'm older, not really old. I'll accept it as something at a regular visit if it is offered, but I won't go out of my way. As I get older, I'll probably get it regularly and if someone in my direct household becomes at risk, I would get it to protect them. But as a single, young person I don't think there is a real strong case that just because you have D, you have to get the flu shot.
Besides, I take so few actual "sick" days off from work, I actually enjoy a good "sick" day once in a while.