On Suicidal they try, but do not take advantage of military starbases to great effect.
What they need to do is either:
A) Upgrade military bases with defenses and offensive capability and have some ship enhancing capabilities
or

Go all out ship enhancing capabilities, and use fleets of ships to guard them.
Also, the AI needs to realize that my fleets hanging out next to his military starbases and planets are not there to say hello, but are indeed a military force that will cripple them in a single turn. And they need to realize that two overlapping military starbases are strategically way more than twice as effective as one.
I don't know if the above is possible. If it's not, then I say drastically reduce the effectiveness of offense and defense bonuses from military starbases because the AI just cannot compete.
On a tanget now, but the AI really needs to stop declaring war several turns before they actually decide to attack. If I sound overly critical, it's because this is such a great game and I know it's going to improve with patches.