Higher resolutions are only necessary when scaling up or blowing up images. A 35 mm slide is about 1' x 1' in size. If scanned at 2000 dpi at the 1' x 1' size, blowing it up to 2' x 2' would result in 1000 dpi, scaling it to 4' x 4' results in 500 dpi and a 8' x 8' final size would result in 250 dpi. And since printing at 133 lpi requires about 266 dpi, you would have enough information. Looking at input resolution can be misleading too, because there is true resolution and interpolated resolution and most companies quote interpolated.
True resolution is the actual or optical resolution captured.
The CCD chip itself determines the true horizontal resolution while the stepper motor that moves the scan head dictates the true vertical resolution. To create interpolated resolution software looks at two adjacent pixels in the image and averages them to create an intermediate value. However interpolation is useful for linework scans because it can smooth the jagged edges on curves and diagonal lines. Another issue is how many bits are captured and how many are used. Most experts agree that eight bits per color is enough to produce good-quality output.