I have 2 cats. If they had an ailment, that I could treat without breaking the bank, or require nursemaid treatment every hour, I would do it. If they were suffering I would have to put them down.
But...diabetes is'nt like the cat was mauled and crippled by a pair of german shepards (like one other cat we had) and suffering. We tried to keep him comfortable, but decided it was best for him to have him put to sleep.
If you love it...treat it.