On the other hand, salaries for most starting attorneys (i.e, associates) are fixed nationwide and proceed, generally, in lockstep, so almost all firms will pay the same, regardless of technical skill.
This is not true at all. Salaries for starting lawyers at "BigLaw" firms are generally fixed...but only by firm and by location. So Firm A in NYC pays $XXXk but their LA office only pays $YYYk. Firm B probably pays close to $XXXk in NYC but might pay $ZZZk. And if you go down to the smaller firms, they could pay anywhere from $40k to $XXXk based on location, type of law, etc.
After the first year, very few firms are lockstep with regards to raises...in the past few years, I think only 1 or 2 major firms is still on the lockstep model. Raises after 2nd year are almost universally based on performance, which depends heavily on technical skill (technical meaning legal, not programming, skills).