Totally agree...if you are going to do trigger points with a 30g 1/2inch needle - it is totally worthless.
To get meaningful results, you need to hit the facial planes, and the deep ones - it is a very targeted and specific injection. My bias is that since I have been targeting specific structures (deep facial planes - and serratus muscle), the effectiveness has improved.
To answer your question, 2inch is for ease of ultrasound.
To see the needle effectively (using an in-plane approach), one needs to make sure the angle between the ultrasound beam and needle is less than 30 - 40 deg. To accomplish this, you need to enter the skin farther away from the probe. The payoff is you get to see the needle very well. The cost is you need a longer needle.