Often, people post something like "I have 500 active projects! There's no way I can just pick actions out of my NA lists!" In that case, trimming the list down to something manageable is essential. Especially since these people are often new to GTD, and generally overwhelmed by whatever mess inspired them to try GTD in the first place.
Once your life is more under control and you are more comfortable with the system, then you're the best judge of what a manageable load is. If you're comfortable with a longer project list, go for it. Just make sure you aren't becoming overwhelmed, and take action to trim back if needed.