But it's an intranet, we know who uses it - First off, employees change and their circumstances change too. If you break your hand tomorrow, you'll be very glad of the keyboard navigation. If you lose your glasses, suddenly those tiny fontsizes are a pain. More to the point, did you bother to ask your users? You might be surprised to discover that Bob in accounts is colourblind, or that Joanne never bothers with the intranet because she can't read the very light text at her age, or that the marketing department suffer from mild Dyscalculia.
Blind people don't use X (where X is whatever service being provided) - All wrapped up in this one is the misconception that accessibility is just about one usergroup and the mistaken belief that people with disabilities don't need access to services online. Look, even if a person with a particular disability doesn't need your service personally, they might want to use it for others. I've seen more than one complaint from, for example, a blind user who wanted to buy a skateboard for their nephew but couldn't due to a poorly implemented site.
It costs too much and will take extra time, we'll add it later - Generally missing the fact that you were updating on a regular cycle anyway, so it would have been easy to build it in over time. Plus any initial burst of effort will save you time in the long run (say when your branding gets changed and you only have to alter one stylesheet). Besides, on a very basic level, accessibility is something you have to actively remove through design decisions. This excuse is often heard when a supplier is trying to not be made accountable for their lack of knowledge, or wants to charge for additional services.
Accessible sites are ugly - No. Poor design is ugly. Accessibility is just a set of guidelines to work within. You can create sites of great beauty that are wonderfully accessible. There are plenty of great designers who can do it for you if your own aren't up to snuff.
Nobody's complained - No, it's true. They all went elsewhere instead, as is people's standard reaction when something on the internet doesn't work. This may account for your failure to turn page views into conversions.
So, those are my pet hates. Which excuses for not doing accessible design do you particularly dislike?