This morning, as I was walking to the office, I saw an advertisement on the side of a bus for a digital advertising company. The ad wasn't great, but it was clean and clear. I was intrigued enough to call up the website on my phone. And... well... apparently they didn't want me to really look at their site.
They didn't actually even have a mobile site - it was a brochure-ware site that wasn't mobile-optimize and really didn't tell me much about what they did. But then I flipped my phone sideways and, well, I was effectively prohibited from viewing their site for reasons, from my quick trip to their homepage, made no sense to me. I often view websites with my mobile device in this orientation. It's big enough to see plenty and it allows me to see the text at a larger size. And if I want to watch a video, it's going to open in this orientation anyhow.
So why did they do this? I can only speculate: They paid some website to do the work for them and the coder was lazy. They have some specific content that's really long that they want you to see all at once. They actually don't care about mobile.
I certainly wouldn't advertise my tiny consultancy on the side of a bus (it wouldn't reach my target audience, I don't have a marketing budget, I'm not trying to play at that level, and so on.), but the bus advertisement lends them a "next level" of credibility and if you advertise on the side of a bus, you have to assume most of your audience will be visiting your website on a mobile device. Instead of allowing me to learn about their brand, I have a negative impression and I don't see a reason to spend any more time on them.
I can't find any stats on portrait vs landscape in a quick Google search, but pretty sure anecdotal unscientific research would suggest most people do use their phones mostly in portrait, but that's no reason to shut the door in the face of a potential customer. Audit the experiences your customer will have. Are there any places where you're keeping people away who want to get to know you?