why 3000 hits still matters

Or, "why Bill Simmons shouldn't drag the 3000 hit milestone down with the steroid era meaninglessness of 500 homers".
In terms that Simmons can understand, just because Judd Nelson peaked "when over-the-top performances in enjoyably cheesy movies were all the rage", that doesn't mean you can throw out John Cusack's career too, or even take away from the 80s resume of Curtis Armstrong. It's the difference between being in the right place at the right time and being very good consistently over a long period of time. While Simmons may consider a 20 year span the right time, part of the point here is that there aren't a whole lot of players who last 20 years in the majors, even in the medical miracle age we now live in.
The problem, of course, is that he's right about the 500 homer mark. There are currently 4 active players over that milestone - in the 1971 and 72 seasons there were 5, but we are seeing unprecedented numbers of players on the doorstep. At the end of 2004, 10 active players were over 400 homers, the most ever. Granted there are more teams and more players now, but there is only 1 active player over 3000 hits - Palmeiro. And there is no great mob of players on the doorstep - there are 2 others over 2500 (Biggio and Bonds) and Julio Franco at 2497. In the next 10 years, even with steroid testing, we may see another 10 guys pass the 500 homer mark, but after Biggio and maybe Bonds (if he ever comes back), it will be a long wait for the next 3000 hit player.
As for playing in small parks, sure that helped Palmeiro's home run numbers, but through 2004 he had 82 more hits on the road than at home, and he had more road hits than home hits in all but 6 seasons. Palmeiro had more hits than Wade Boggs in every season from 1990-1999.

Take a look at the other people who have broken the 3000 hit mark recently - Ricky Henderson, Cal Ripken, Wade Boggs, Tony Gwynn. Biggio should be next. If Barry Bonds does it, I will be fully prepared to argue that at least 250 of his hits were steroid fueled pop flies, and I won't feel hypocritical about doing so. These are all people who played at a level of excellence for a long period of time, not people who put up numbers for 5 years that would have been record breaking 10 years before. We aren't seeing yearly assaults on single season hit records - that's just one guy, Ichiro, and he's not exactly the picture of steroid juiced power hitters.
That's why career numbers matter - because lots of guys can get 150 hits in a season, but not that many can average that for 20 seasons. The home run hitters broke the formula, because it's still rare to average 25 home runs for 20 seasons, but it got a lot easier to average 40+ over a ten year career. The guys willing to sacrifice longevity for huge numbers at their peak were having peaks that could match the career numbers of the 20+ year players. A player getting 10% of the way to 500 homers in a single season happened 18 times before 1995 and 18 times since. Nobody gets 10% of 3000 hits in a single season. Ichiro, Wade Boggs and Darren Erstad are the only players since 1930 who have gotten even 8% of 3000 in a single season.

Ignore home run totals if you want to. I am. But don't throw out the rest of the career stats as tainted along with them. They still matter.

Especially plunks.