True Peapody. I've read some of your other posts, you seem well organized, you've traveled some distance with your 15" too. If I remember correctly you use an Incase bag?
Haha..sorry for this long post.
I noticed you owned the X301 and the MBA (I have too, along with a 13" macbook, and a 15" MBP both unibody and non). I buy and sell lots of computers (mostly Asus and Apple) so they go through my possession a lot.
I used the x301 mostly for school because it is a windows machine, and that 13" screen size is ideal. It doesn't block the people behind me at all but to be fair it is the same way with the 15". Once I got my 15" MBP (nonunibody) however, I FELL IN LOVE WITH THE ABILITY TO MULTITASK. People will say spaces helps etc, but its annoying to flip from space to space when you have webcasted lectures on one side of the screen and notes you are typing up on the other. Eventually I used the macbook pro so much I took that to class everyday for a while and then alternated bringing the 13" notebooks and the 15" to mix it up.
I don't think you are in school so this part might be of more use for you. Pretty much all of the circumstances you listed in your OP, I have done with the exception of taking to a cottage - never been to one!
After classes were over, and rotations began (I am in pharmacy school) I had to start bringing a computer every day to work. I work at different hospitals and regularly go on rounds and give presentations and do spreadsheets and etc. I also had to do a lot of photoshop work and brochure design for various projects. So eventually I weeded out the x301 (and with that the MBA and 13" macbook because of screen real estate and even LOWER resolution). Now at these hospitals I do not have an office but even so, the 15" has been working great for me so far! ; Sometimes I had to sit in my boss's office with my computer on my lap for hours at a time taking notes and developing ppts. I also had to cram myself in a corner of retail pharmacies with my computer, and there is no space back there at all. No problem. I also started investing more into my current 15" because it started to become my extra limb. I purchased retail Windows 7 and Creative Suites for it. Purchased SSD for it. Purchased extra ac adapter for it. It went with me everywhere. I take it to starbucks and panera where I study and do work, and it fits on those tables just fine. I just took it home from Baltimore to San Jose, CA and back - and surprisingly it fit comfortably on the tray table haha. My 15" is now my main computer, and I don't even connect it to another monitor (I have an iMac for media center purposes only).
Additionally - in bed it is a joy to use and do work on lol, as you can see from the attached pic.
So having gone through using the 13" and 15" extensively as a travel computer, I don't think I could go back mostly because of that screen. Not only is the res too low, its too small for me. I had the incase messenger in medium, and that is an amazing bag. If you own a dslr - it is perfect for the dslr and then 15"mbp. I sold off the bag because I eventually found a really nice slim bag that holds the essentials without looking too much like a laptop bag. Also, the thinner light weight material of my new bag seriously cut down the thought that I was "lugging" my computer around. If you are going to travel with the 15" I suggest getting something on the slimmer lighter side, and it will seem like nothing carrying it around. In addition, I carried my 15" in a women's professional tote with ease.
So in conclusion, it is perfectly possible and not totally uncomfortable to make your 15" a travel machine. I guess no matter which size you choose - just go all out with it and choose the right bag.