There are two main reason why you should make your websites and apps compatible with iPads and other tablets.
1: Future compatibility – the market share is growing
If you look at the usage stats of most sites you’ll see the number of people visiting from tablet devices is slowly but steadily increasing.
The percentage is still quite low in relative terms, and you might not normally focus on such a small subset of users. As it is growing however, with no signs of slow-down, you can be sure that at some point in the future you will be asked why your site doesn’t work well on tablets.
2: Acceptance from influential early adopters and opinion leaders
Although the percentage of users is low in terms of browser stats, the people browsing on iPads and other new generation tablets are likely to be the early adopters & opinion leaders.
These are the people who’s opinions are respected and requested by others. These are the people you want on your side.
If your site / app works well on the new devices you are more likely to be remembered favourably and recommended to others. If your site goes so far as to surprise & delight then even better.
If your site fails, you will instantly be forgotten and dismissed. Even worse, you may be publicly derided.
Supporting iPads is arguably more important than supporting old versions of IE, even though the latter may have a higher percentage of use on your site.
** Disclaimer: iPad ownership does not directly make one a thought leading, cash rich, decision maker.*