The seams or seamless controversy rages on while I sit back and wonder why everyone takes such a narrow view? I see rants in blogs and on Ravelry defending a single position. I get asked to proclaim a definitive preference by many of my students and friends. Sorry, I don't have one! I think every method of construction has pros and cons. I feel it is better to assess every project on the basis of it's individual needs and make a carefully, considered decision.
I also hate the way knitters on each side of the argument use scare tactics on newer knitters. Here's a modified (to protect the knitter) quote from a seamless defender. "Imagine knitting a gorgeous garment without the stressful pain of sewing and seaming".
I think it's wrong that some knitters choose projects by deciding what they don't want to do or more importantly don't want to learn to do! You learned to knit, so I have every confidence that you can learn to seam. There is nothing to be afraid of!
Here's my pros and con's list for each method.
Seams create stability, the knitting is firmer with a seam, it is less likely to stretch in length.
It supports slippery yarns with drape and inelastic fibres. Even Elizabeth Zimmerman is said to have liked phoney seams for the added structure they give seamless knitting.
Shoulder seams and a bound-off back neck will prevent drooping shoulders and sagging necklines. The weight of the sleeves is balanced by firm shoulder joins.
Seams can simplify construction, when working flat you may not have to work multiple shapings all at the same time. As an example you will only need to maintain stitch patterns at two edges. On a cardigan knit seamlessly in a stitch pattern you are working on the two fronts, both sleeves and the back all at the same time.
More detailed fitting is often easier with flat pieces. This can be advantageous if you are a knitter who needs many adjustments.There are more places to put shaping.
Seams allow for shaping by easing in the fabric, as an example to create a rounded sleeve cap.
Seams can be a design element.
Blocking is easier when working with flat pieces.
Garments hang better.
There are places to weave in ends.
For instructions on how to seam go here.
Seamless (Seamless garments can be worked flat or in the round, top down or bottom up)
No interruption of patterns by seams
Working from the right side makes patterns are easier to follow when there is action on all rows.
No purling in stocking stitch means no rowing out. (Rowing out happens when knitters have a looser purl than knit gauge.)
Working this way helps with three dimensional thinking.
Top down seamless garments can make checking lengths easier.
Less finishing is required.
There can be twisting of the garment when it is worn. Working in the round occasionally creates bias in the fabric.
The work is less portable as the item gets larger and heavier.
Garments which combine flat and in the round knitting may have gauge differences.
There are no reverse shaping instruction.
There is more to tear out if something goes wrong.
The additional weight of the work can be a problem for knitters with repetitive stress injuries or arthritis.
The gauge swatch must be knit in the round if garment will be created that way.
If you have additional pros and cons please leave them in the comments.