The reason for the trash cost that gets boosted by 3 for each hosted card is to make it unappealing to trash the RecStudio in order to trash both the installed cards. To be that way the sum of the trash cost of the installed cards has to not exceed the trash cost of the RecStudio with two cards on it, which is 9.
Here's the division in assets (up to Free Mars of Red Sands cycle has been released at time of writing):
Trash cost 0 (10 cards)
(Not shown to keep review small and it's not really relevant)
Trash cost 1 (9 cards)
...
Trash cost 2 (26 cards)
...
Trash cost 3 (42 cards)
...
Trash cost 4 (22 cards)
...
Trash cost 5 (15 cards)
Alexa Belsky,
Broadcast Square,
Corporate Town,
Cybernetics Court,
Director Haas,
Eve Campaign,
Genetics Pavilion,
Hostile Infrastructure,
Ibrahim Salem,
Jeeves Model Bioroids,
Marked Accounts,
Private Contracts,
Pālanā Agroplex,
Victoria Jenkins,
Worlds Plaza
Trash cost 6 (3 cards)
Chairman Hiro,
Illegal Arms Factory,
Reality Threedee
Trash cost 7 (1 cards)
Trash cost 8 (1 cards)
While some 5 cards are not uncommon (Eve Campaign, Hostile Infrastructure, Jeeves Model Bioroids) the 6 and up cards are pretty rarely seen in actual play. And the 5 cards are not the kind you'd normally put together in NBN (or what you'd inport a •••• card for in HB). So I think it's safe to say that it's extremely unlikely that anyone will ever put two cards in a Rec Studio that together will cost more to trash than the RecStudio.
This means that RecStudio doesn't add risk unless the runner has a card that lets them trash an asset but only one asset. The only card that comes close is Drive By but that only works on unrezzed cards. Derezzing is also harmless, the cards hosted on it remain hosted (according to FAQ 4.0).
While the RecStudio doesn't add risk is does cost a deck slot, 2 to rez and is positional (it has to be rezzed before the assets and agendas to throw on it). That's still quite a few downsides, is the effect even worth it?
Frankly, I doubt it. Theoretically the benefit of installing multiple assets and/or agendas in the same server is that you only need a big chunk of ice in front of it to keep both installed cards safe. In practice runners get into the most heavily protected servers, especially with the rather weak ice that NBN tends to employ. And once in the runner simply can kill two birds with one stone. You just end up painting a bigger bullseye on your server.
There are of course janky plays. Since the runner has to access all cards when accessing you could do stuff like a double Edge of World server.
You would think there are some nasty sysops who get to play in the RecStudio as well but all sysops are upgrades. Characters, which are assets, are also harmless when accessed. Executives aren't traps either, except for Toshiyuki Sakai if you squint your eyes a bit.
I simply fail to see a realistic use case for Full Immersion RecStudio.