It's definitely a personal choice and theres no real reason you should feel you need one. For me, I got one from my doctor at 20 weeks because we have some family history that they wanted to do a thorough check with.
I did end up paying for one at 26 weeks, and I don't regret it at all. This is my first baby, and I feel like it's a once in a lifetime opportunity. It was also a way to get family to bond with baby, as my husband was there along with both of our parents. My MIL never saw an ultrasound before, so she was blown away. it was worth it to me, but insurance didn't cover it so I had to pay for it. But I got tons pf pictures and the DVD of the whole thing so for me it was worth it.