While slightly outside the scope of the original question, Runners do have a good reason to own a very diverse stock portfolio and that's information gathering. They'll legitimately receive periodic reports on what the company is up to with info that doesn't generaly make it into common knowledge on the streets without needing a contact to find the info. Beyond that, if they manage to build up shares that have votes, they can gleen valuable intel from how they're courted and by whom for proxies on their votes. If they can manage to build up a signifigant amount of stock, in the whole percentages, either individually or through a closed investment group, they'll be able to pull in even more valuable intel. Situationally, they can also build up signifigant holdings in smaller businesses that have regular dealings with much larger ones, pulling information from there and even gain legitimate access to facilities they'd be barred from normally.
I forget which book it is, but there's also mentions of a "shadow exchange" for the SINless, which is very useful for dealing in the markets and gaining information access like the above.