Wear and tear on the inner fabric is actually greater than the outer shell fabric...every time you put it on or take it off, stretch your arms, etc, you're abrading those fine denier yarns. It's pretty common for jackets to use a slightly heavier fabric choice for liners. That said, we're still talking 20d. Without knowing specifics about the fabric you can't say that "20d" is any more or less breathable than another 20d or 10d fabric choice, but if less breathable that does help some to retain warmth a little longer and it's still plenty able to pass moisture unless it's coated or just an extra dense weave pattern/thread count. Pullover is smart for a touch of extra warmth but that "fashion baffling" and what appear to be very loose wrist cuffs both help to take away from those gains, imho. Yes the seams are sewn through...don't see box baffling until you get into some very thick down parkas.