Of course there are. Some. And the reasons are the same for the forces as life in general.
The main one is: most people don't change their vote. Where I live, Margaret Thatcher would get elected if she wore a red rosette and said, "I have a social conscience". So would a donkey.
If that were not the case, marginal constituencies would be unimportant at election times.
The forces are another type of constituency: for them, read Ken Livingstone and donkey. On the whole.
Tin hat on and waiting for all the "I weigh up the issues every time, and vote as necessary", responses.
CG