I bet there will be a 10.3.10 some day.
Re upgrade rates... if you bought the last OS recently, you're free to keep using it. Your Panther apps running in Panther will keep right on working. If you find Tiger has value, you can get it--I'm glad I did!--but that's your choice.
OS X is a new (well, young) and revolutionary OS. Any new OS will grow and improve rapidly at first, and then that slows down over time. This is happening, naturally, with OS X, and Tiger will be with us even longer than Panther was.
And these are big upgrades. The decimal point is just a naming convention to let Apple keep using the "X" logo for branding.
To put the upgrade cost issue into perspective:
Since OS X 10.0 was released in early 2001, there have only been THREE paid upgrades of OS X ever: 10.2, 3, and 4 (remember: 10.1 was free). The time leading up to those paid upgrades was 17 months, 14 months, and 18 months. NEVER 12 months as people often falsely repeat. The next one is expected to be even longer in coming--some time in 2007 maybe.
10.0 Cheeta
...17 months (includes free 10.1 upgrade 11 months before Jaguar)...
10.2 Jaguar (first paid upgrade)
...14 months...
10.3 Panther (second paid upgrade)
....18 months...
10.4 Tiger
....even longer before the next one?
If you'd rather Apple not improve things so much, and release big upgrades every 2 years instead of every year and a half, you can achieve that by just not buying every upgrade. Lots of people get by just fine that way, just like they don't buy every version of every software program they own. Even NEW apps often run on older OS versions... and the ones you already have will of course keep running.