Originally Posted by
jccaclimber
Read this one again if you didn't. Supporting a length of the seat tube is important for much the same reason following "minimum insertion" marks is important. If you look around the forums you'll find a few threads where people didn't follow that guide and separated their frame at the seat post lug.
The biggest problems with using a shim, is that modern bikes now often have the seat tube extending far above the seat stays, where as in the past the seat tube stopped at the stays. if your frame doesn't have an extended seat tub, a shim will be fine, would use caution if it does as you may end up with too little fully supported lenght in the seat tube.