I agree that more and more publishers are doing it, and it is a good thing.
Remember, just a couple of years ago, there were NO d20 rules for Mass Combat, or Ships, or dog fights, or stone age adventures, or horror or a lot of other stuff. Then we went through a period where several different companies tried to address these needs, and did so with varying degrees of success. Now we are in a stage where the cream is rising to the top.
Those rule sub-sets, feats, and spells that are good are being reprinted and built upon, some even endorsed by WotC. Those rules and ideas that are not so good tend to be left behind.
I anticipated this a long time ago, which is why when I started out, I wrote books full of crunchy rules that could be easily exported to any other game or to any other published work. Today, however, my product lines are moving more towards intellectual properties, campaign settings and other cool things that you can build with all the cool d20 rules we have all made in the past few years.
In fact, I have been looking forward to this time since 2001. I knew the first few years would be a tough slog through a lot of aimless spells, feats, prestige classes, etc. But, I used to say, once there is a bunch of that stuff out there, we will be able to sift through all the best d20 rules, pick what we like, and build totally cool new stuff with it.
That is exactly what
1948 is. It incorporates several rules from other publishers. Why should I waste my time coming up with rules for Dog Fighting when the guys at RPG Objects already did it? Why should I bend my mind trying to come up with some sort of "leader" PrC when the Game Mechanics' "Commander" is just what I'm looking for (with a few tweeks, of course)? I'd rather use the pieces that are already invented to build something new. Why re-invent the wheel when I can use a pre-existing one to invent a car?
It's the best thing about d20, if you ask me. It helps the game in *so* many ways. It saves the game designers a lot of time, and it makes the best ideas available in every book.
On the other side of the coin, there are some publishers, especially new ones, who still want to re-invent the wheel. I’m not going to speculate on their motives.
Consider what things will be like in a few more years....