Here are some thoughts...
The vibration from the birth name never goes away - it's always part of who you are. You can add, delete, change or whatever, but the new energy remains "modifying" energy. With that said, I think part of the significance comes because anytime someone changes their name the new vibration does affect them. If you've ever been married and changed your name, you know what I mean. The name change that occurs with a marriage isn't always beneficial, but the name does give an outward expression of unity, so it serves an important purpose (I am not judging people that choose to keep their maiden names). However, usually marriage names are not "chosen" - you get the name of the person you marry - good or bad.
When someone changes their name for other reasons it is usually done with INTENTION, and anything done with intention is powerful. The birth name still provides the foundation - and the new name, depending on the INTENTION, provides modifying energy (which can be very powerful, especially over time).
The ommision of a middle name, unless it is legally omitted, would not change the way a chart should be read - with the full birth name.
The legal name change would be worth looking at though - it that case, I would read both charts, and give more influence the longer the name change had been in effect.
