Except for one particularly fastidious GM, I have never seen this rule implemented. I have seen tables talk about this rule, which partially guides the answers below.
And of course, some people somewhere do use it, but there are a few good reasons it is seldom used (in my experience):
Handwaving Magic Item Identification
One reason you don't see this rule come up often is because many tables skip the process of identifying magic items. In some games (especially older editions of D&D) identifying magic items was a big deal. The first step would be to discern whether an item was magic or not. How do you tell? Maybe the item has runes or another script your character can read, perhaps you recognize the particular item, etc.
This rule helps with that process: 30% of the time you can tell when an item is magic because it is completely obvious.
Many GM's hand wave this part of the game because it quickly becomes tedious. Players want their treasure and don't want to problemitize the potentially lengthy process of determining which weapons are magical, and what they do.
Quick Item Generation
A second reason you don't see it come up is because of how GM's generate magic items.
More than a few magic items are generated on the fly, using the tables in the GMs guide or something similar. This lighting rule isn't on the table, so typically it is glossed over in the fray of the game.
Player Inconvenience
This rule is often inconvenient for players. It's nice when the rogue gets a +3 dagger, it's not nearly as neat when the item is impossible to conceal. Especially in low-light situations, this kind of item can be a large detriment to the party.
