I had a customer bring in a trolling rod with a loose reel seat. I tore the butt off to make the repair and found that the blank had been cored from the gimbal up to the reel seat with a solid fiberglass rod. I've never seen this before. I this common place? Really needed in a #30 rod?

Sometimes used to extend the rod length as the person grows so the rear grip is longer. it depends though.
i believe its sometimes also done to give a tad more power to the rod.
Hope that helps!

Is it a rack rod? If so I have had guys bring in rods for repair and found the same. Some companies do it so they can use the same blank and just change the configuration of handles guides and blanks. Handles are put together in one department blanks and guides in another. Then mated as needed to fill order requests as they come in by mating based on what is requested.

It's a common practice in commercially produced rods.

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Old time builder can remember when aluminum tubing was used to extend rod lengths in glass rods. This was way before Titanium butt sections say back in the 50's

Good Wraps Bob