I love the combo of a 15" MBP for short-haul trips or working from home and the 13" MBA as a road-warrior. I have tried to have just a 13" MBP instead (1 machine instead of 2), but the tres-glossy screen and the extra 1.5lbs of weight compared to the Air made the tradeoffs not worth it.
For me, the 13" Air is almost 1/2 the weight of a 15" MBP (2.9lbs vs. 5.6lbs). That's a huge difference. My MBA can be with me all day and I barely notice it. (I've several times over the last two years had to re-check my bag after leaving somewhere - just to be 100% it's in there!). If I have my 15" MBP with me all day - I feel the weight, and I know I'm carrying around a beast.
Performance-wise, with an SSD, the RevB/C MBA's feel just as snappy as their 15" brothers, at least for basic tasks. The only slowdown is the 2GB RAM limit and the fact that the SSD's in the RevB/C models aren't as fast as current SSDs (but way above those in the RevA MBA!
).
IMHO, the 13" Air is the perfect travel companion to a larger laptop or desktop that's primarily stationary.