Worst problems with larger trees and shrubs typically date back to when they were small trees. The only difference with the specimen being more advanced is that you have more to work your way back through to look for a tight knot or fist below the crown dating back to when it was left too long in a band or small pot - buying them small only makes the deformed roots easier to expose and see. If the disposition of the roots is too severe you are either stuck with planting it and hoping for the best or giving up on it - I threw out a 35 dollar one gallon weeping Nootka cypress because at planting I found that the ENTIRE root system consisted of what looked like a ponytail hair braid like some women belonging to Jehovah's Witnesses wear. How the grower was even able to get the rootstock to twist around itself like that I cannot imagine. Not only was it coiled but it dipped down and then back up, in a U shape.

You only plant in spring because that is when the plants are on the market (in good condition, apart from the usual root issues) or because the plants are tender kinds. Otherwise hardy stock should always be planted in fall, late enough that summer heat and drought has passed or is waning but early enough that the big fall annual root elongation has not occurred. (If the roots have to be operated on to a sufficient extent most of the extending of existing roots in fall may be lost, as only intact roots do this). You can tell if a specimen may be about to do this or already doing it by looking at the top, the setting of winter stem buds at the ends of mature, hardened shoots is what causes the root tips to grow at this time of the year.

Spring planting is culturally inferior because the soil has been chilled all winter and summer heat and drought are around the corner. Bare-rooted stock from grower warehouses is only planted in spring because the feeder roots are all dried up and dead, need to be replaced before the specimen is fully functional - fall-planted bare-rooted stock is essentially hardwood cuttings with old roots already present but still needing to be protected from the worst winter conditions. New roots grow in spring in response to the opening of winter stem buds. Both in fall and in spring these buds generate hormones which cause the roots to grow. This is one of multiple reasons you don't want to cut tops back at planting, you are reducing the amount of hormones that will reach the roots.

Holy Smokes, thanks for the education, Embothrium.

"I read you can plant potted at any time, though spring and fall are better depending on species."

That is basically correct. Most pot-grown trees (not really the same as "potted trees") can be planted just about any time the ground is not frozen, IF (as you alluded to) the root system doesn't require major manipulation. When I plant pot-grown trees for myself, I virtually never completely bare-root them, but almost always loosen up the rootball to the point that I can ensure the roots are not wrapped around and girdling. In other words, I leave a lot of potting medium attached to the small roots but get rid of a lot of the extra potting medium and correct root structure problems.

Most trees (but definitely not all), whether pot-grown or otherwise, can be planted in moderate and warmer climates (about zone 5 or 6 and higher) anytime between mid fall (after leaf drop) and early spring (before bud-break) as long as the ground is not frozen. I generally don't do much personal tree planting in late
spring or summer, even when I'm working with pot-grown stock in good
condition. Some trees, particularly some with thicker rope-like roots, do much better and have a significantly higher success rate if planted in spring, rather than fall.

I forgot to leave this link to what I believe is a pretty good planting guideline:

Planting a Tree or Shrub

Thanks Brandon. The expertise here would have saved my bacon if I'd know about gardenweb last year.