Spending at least as much time aboard at berth as compared to "at sea," we prefer to be in sunlight and in free breeze.
There is certainly something to be said for keeping a boat under cover. Particulary an older boat like ours with teak decks and a rainforest of external teak trim.
Our marina has a pretty big bunch of boathouses, some of them with boats 50 feet long in them, although most of them are more in the 36-46 foot size. All the boathouses are in our basin of the marina, and one thing we've noticed over the last 16 years is that while the boats in the boathouses are generally in immaculate condition, they don't get used much. Some of them will be gone in the summer for a month or so, and others get used for weekends during the boating season. But come fall, winter, and early spring, most of them are shut up pretty much full time.
Also, the boatrhouse boats don't seem to get visited much in the off-season.
So I think a boathouse is a great place to keep a boat if one isn't going to use it all that much. It stays clean, the brightwork, canvas, decks, etc. are all protected, and short of heat in the colder times of year, the boat can simply sit.
The other thing we've noticed is that the boats that do get used year round, be it going out or the owners staying on them for a weekend or whatever, are pretty much all kept outside.
I'm with Mark. We use our boat all year. We go out when we can, but often the weather prevents us from going out in the winter. But we'll still go up and stay on the boat for a weekend, or sometimes just for a day. We would not do this if the boat was in a boathouse. It doesn't strike us as being very interesting, sitting inside looking at the walls.
Our slip is near the outer edge of the marina and we can see all of Bellingham Bay and a bunch of islands. We can watch the clouds and sun and sunsets and the weather. It's basically like having an inexpensive getaway cabin with a million dollar waterfront view.
The downside is the boat needs more external maintanance than if it was shed-kept. So we have covers on as most of the external teak and we keep after trying to keep the boat reasonably clean. If we didn't use the boat as much as we do, being outside would take its toll. But given the way we use it, it's definitely not neglected.
And it's fulfilling what we bought it for, which is something we can use and enjoy year round, regardless of the weather. If it lived in a shed we probably wouldn't bother to come up to it at all except to take it out periodically when the weather and our schedules matched up.