What mechanical implications exist when allowing duplicate stronghold types in a castle?

Matt Colville’s supplement Strongholds & Followers details, among other things, rules for building optional strongholds and castles, with castles being complexes of multiple strongholds.

The strongholds are divided into four broad types: keeps, towers, temples, and establishments. A stronghold (of any variety) can be controlled by only one PC at a time, and can only offer its most direct benefits to that character. On page 11 of Strongholds & Followers the rules state that castles cannot contain more than one of each stronghold type.

A castle cannot contain multiple of the same type of stronghold. If you build a castle with two towers, for instance, only one of them grants the benefits of spell research. Also, a stronghold’s benefit only applies to one character at a time. (Strongholds & Followers, page 11)

I am running a game using these rules in which two PCs (a Wizard and a Sorcerer) will almost certainly want to build a tower each. Even if they don’t, the party has no use for a temple (or variant on a temple, like a Druid’s Grove). With four players that means that unless there is at least one duplicate stronghold type, at least one player will be left out. This is easy enough to just overrule (I plan on allowing two towers), but I’m curious about why the restriction on duplicates exists. I have not found anything in the book itself.

Is there a mechanical implication to duplicating stronghold types within a single castle?