People don't like to make multiple entrances for several reasons:
1) They have to defend every one, somehow.
2) One good entrance is "good enough"
3) They are afraid they'll forget and leave the back door open... like last time... 
If you don't buy in to that logic, then there's no reason not to have more than one entrance.
I have multiple entrances,
but I'll admit I only feel perfectly safe with the additional ones if they're a "controlled area" - walls or (more often) terraces that have been sculpted to remove all slopes and access.
Imo, a long straight valley is a great entrance for a couple reasons. First, it looks cool - yeah, but there is an element of aesthetics in this game. But more, it gives you great control over your entrance, especially if you reserve the first couple z-layers under that valley for tricks and traps.
Siege engines can only fire along their z-level, and they shoot far, and ballistae shoot thru multiple enemies - so, you're creating an easy funnel that channels all the enemy into a thick mass that engines can fire into - nice. The side walls are perfect for fortifications and x-bow stations, and since there are levels within those "walls" you have areas to build barracks and practice ranges and whatever - convenient.
(The path will take a dive into your fortress at the end of that valley, under a "balcony" that has the siege engines watching over it, and shooting down the length of the valley. There are other threads and some good wiki articles on this.)
If you're going to build a death-trap, you have both an area above the target for the liquid, and the containment pre-built - handy.
The only drawback is that, when you send dorfs out into the wild world to cut trees or hunt or mine distant hills or whatever, the entrance is not where it comes out of the ground, but where the valley exits. Which gets us back to your question of multiple entrances.
Lots of approaches, lots of solutions. Have fun!
