Agree, 1/4" shims make the pockets play funny - not just a little, it's quite noticeable.
In my case I was restoring an old table I'd never played before, and didn't know if I'd like it. So I couldn't justify the expense of shipping the rails to someone, or getting a specialist to travel here and do it. And at the time, I didn't feel confident enough to extend the subrails myself (I feel differently now, I should have just sacked up and done it).
If you already know you like the table and how it plays, pay the extra $ and do it right.