All penetrations require reinforcement of the hole that is cut in the tank shell. The reinforcement can be provided by several means, including excess tank shell thickness beyond design, the thickness of the neck, a repad, etc. There is nothing special about a manway in this respect. Due to the small size of these tanks ( shop fabricated ) it is possible that only the manway needs a repad.

I am familiar with the requirements of reinforcing shell openings. My question is if anyone else frequently sees unnecessary reinforcement added to manways.

Take for example a tank that is 12' Diameter 16' tall. The required minimum shell thickness is 0.09"
We would fabricate this from 1/4" plate giving us 0.16" excess shell thickness for reinforcement.
Yet, the concept sketch from the customer shows a large reinforcement plate around the manway.

It isn't a big issue to add one if the customer wants to pay for it, but I don't see the reason behind it.

Has anyone else experienced this?

theoretically, you are correct. Practically, most shops will add re-pad without asking when we receive fabricaiton dwgs for review. Is it because the wall is so thin, afraid of local deformation or damage by foreign object so to make it sturdy besides of code calc ?
Costwise, we never care the little extra for a repad on manway. When we review bids from 2-3 bidders, none will raise this issue of charging couple thousands $$ just for a re-pad.

Manways have no external loads so it would come down to.. because it looks right, because it saves performing calculations.. that's about it. Sometimes it is cheaper to over design the tank than perform the calculations when you don't have suitable staff in house. Personally we always do the checks and if the integral reinforcing is suitable we don't add a pad. Even with pressure vessels I have heard from vendors that pads have been included because of their own conservative company standards (whatever that means..).

If you are in doubt how re-pad works, I suggest you to go to ASME B31.1 - Power Piping and take a look at FIG. 104.3.1(D)

Hope this helps.
eng. Faustini