Many people say things like “well, Netscape and Internet Explorer have some arbitrarily high percentage of the market between them, so why not design for them?” This doesn’t wash, for several reasons.
It may help to think of it as “browser environment independence”. Users can turn off image-loading (I do, for example), or change their font size, or shrink their window;
they may have a laptop with a 640×480 screen, or only a grayscale display, or even a one-bit display; they may have Netscape 2.02 or 3.01 or 4.04, or MSIE 2.0 or 3.1 or 4.01; they may use a PC, or a Mac, or a Unix workstation. That “high percentage of Netscape/MSIE users” is a fragmented agglomeration of situations, not an army marching in lockstep. MSIE for the Mac lets me turn off not only image loading, but also GIF animation, background music, and frames. (So “get a frames-capable browser, loser” is not going to get me to “upgrade”; it’s going to get me to go elsewhere. I hope you’re not trying to sell me something…)
There are also other browsers. If people can’t read your page at all because all they see is “[IMAGE][IMAGE][IMAGE]” or a notice saying “your browser sucks, get Netscape”, you’re losing your chance to reach those people. Yes, sometimes you can’t avoid making the page less useful for Lynx users (for example, if you’ve got a photo gallery that uses small thumbnails) but it should still be as usable as possible (use ALT attributes on the thumbnails to say things like “[24K JPEG]”). Pretty “front page” imagemaps should also include a textual list of options at the bottom, a client-side imagemap (which will help newer versions of Lynx, but won’t help Netscape with images off), or at the very least a link to a text-only home page.
Pocket-size systems like the Nokia 9000 Communicator incorporate Internet access, including a Web browser, in a portable phone; if your business might want to attract people with lots of money, “on-the-go” executives and the like, locking out people with Nokia 9000s is probably not your first choice.
Possibly the most important other browser is the search engine robot. With people asking for tips on how to move up in search engines, here’s a tip on how not to move down: write for text browsers, because that’s what search engines most resemble. Don’t believe me? Ask your favorite search engine about “frames-capable browser” and watch the hit count climb.
Second, most pages don’t need Netscape’s, or Microsoft’s, proprietary extensions. The most useful of them have made it into HTML 4.0 Transitional, and when used judiciously won’t harm the page’s usability with older browsers. (For example, using <body bgcolor> to set the background color of a page won’t make the page unreadable on browsers that don’t support it.) The <font> tag is not necessarily safe, however; Warren Steel has written an explanation of why FONT can be harmful as well as the excellent Hints for Web Authors
Third, they’re often used so egregiously that the page just looks ugly. Use of <blink> was the first major offense in this category, followed later by backgrounds (often with bizarre color combinations) and of late by sheer overuse of frames.
Finally, and most importantly, overuse of the layout possibilities (especially frames) offered by some browsers distracts from the content. The content which is, hopefully, the whole reason for the page (or site) to exist in the first place. There are already plenty of low- to no-content “billboard” sites; we hardly need more of them.