Filling out the Pokédex has to be the lamest, most mundane task set before you in any (every) typical RPG in the series. The best thing about Gold/Silver/Crystal (I think) and Ruby/Sapphire/Emerald (I know for sure) is that you don't have to catch anything to progress in the game. I mostly ignore wild Pokémon and spend all my time and money on the ones I intend to actually train. In fact, the other day, I accidentally defeated Groudon with my Castform in one hit and decided I hadn't saved recently enough for resetting to be worthwhile.
You know you're never going to 100% it unless you burn a lot of money on all the games, or actually do have friends with the games needed to help you out, as well as hacker/pirate friends to get you Arceus (oh my, Arceus!!). Knowing that, why bother? Enjoy what you should be enjoying in it.
...That being said, my Pokédexes are pathetic in Gold and Ruby, and when Gold and Silver remakes for the DS come around (I'll want the Silver one, for Delibird (Yes,
I'm sure they're coming)), it probably will be there too. I just don't see the point in spending all the time and money I could be putting towards level grinding and TMs, respectively, on bear traps for bears I never plan on looking at. Don't feel cheated out of a good game. That is, if you like Diamond and Pearl... I personally hated Diamond when I played it, but I'll catch up with new features like Wi-Fi and stuff with "DawnSilver."