Leopard required an 867 MHz G4 or better. Earlier machines ran Tiger and OmniWeb just fine if you didn't push them too hard. My main machines still run Tiger most of the time, because upgrading to Leopard would have required large outlays to upgrade other software that didn't get used quite often enough to justify the cost, but not so infrequently that doing without would have been convenient. I also support a group of friends and relatives who had no reason to upgrade to Leopard, and it is much easier to support someone when you've got a machine that runs the same software. I do also run Leopard and Snow Leopard, and have used OmniWeb since before it was free. I think that was all of your assumptions, wasn't it? :-)
My interpretation of the graphs at
http://update.omnigroup.com/ is that about 20% of the OmniWeb population is using Tiger, and the rest is split pretty evenly between Leopard and Snow Leopard. For my part, I understand that at some point it will not make sense for Omni to produce any more Tiger-compatible versions of OmniWeb, but as it is a free program now, I look at each new version as a gift. If yesterday's version was good enough for my needs it will probably remain useful for quite some time even if it never gets another update.