Leopard comming around soon! So don't spend money on Tiger. It's pointless to now
One good reason to buy Tiger now is if one of the machines you own -- say a powerbook-- can run Tiger but is still running Panther, or is running a family pack licensed Tiger, and you expect to sell it or give it away in the relatively near future.
It's nice to send a machine off in the most usable fashion, no? I like to give away a machine and the best OS (with updates) that it can support and still run decently. That way it speaks well for the capabilities of both the machine and Apple software. Recipients of my giveaways in the past have later become Apple customers on their own. I think that's partly because what I gave them was in good order, easy to reinstall and update if necessary, and not more than one major release behind the times.
And of course you cannot legally part with a machine carrying a family licensed OS unless you also transfer the discs and remove any installations you made on your other machines under that license. So you're not going to do that, right? Since your kids still need their installations? So then with your intended giveaway machine, you'd maybe be looking at re-installing even some old Jaguar that came on it, and passing along those original discs plus the drop-in CPU upgrade discs that only took it to Panther... a sort of yukkky gift and certainly not much of a selling point. I'd buy it the Tiger instead.
I just gave to a neighbor (for his grade school kids) an ibook with a new single user Tiger on it, updated, and threw in a couple of 1st-gen nanos so everything hangs together and will last the kids a couple years unless they turn out to be juvie rocket scientists or rock stars. They're thrilled because they had NO laptop and NO iPods, and I'm thrilled because I have an excuse to check out the new nanos and to daydream about that nonexistent 12" MBP I'd like to get for Christmas. With Leopard installed!