What better way is there to defend your base, than to have no one attack it? Most players will choose to encroach closer to your base, only to stop when there are bunkers and siege tanks. Why not make it harder for them by having more bunkers everywhere. Ok well not everywhere, but at other choke points, so when your opponent comstats, parasites, or scouts with an observer, they'll think twice about attacking, if they see 2 bunkers(empty) with tanks and marines.
Having 1-2 marines in a bunker also helps, since most players won't be able to tell how many marines are really present. This is a good idea to buy time, since nobody likes to lose units. Yhey'll take there time slowly using tanks to siege and unsiege, making it possible for you to get that extra few seconds that you need to fortify or attack.
Usually when you plan to move up to take more ground, it's ok to take all the marines out your existing bunkers to move to a new position. These bunkers act like a backup, to retreat to if the new ckoke point fails.
Sure this tactic is risky, but how else can you hope to hold new island expansions? Building a bunker to scare off would be attackers is a good idea, especially if you have no defense there, but you'll have to build turrets to push the dropships away, since your bunker is empty.
Bunkers also make good scouts. They're cheap to build all over the map, as like turrets.
