Dedicated theme subsites for each function – or jack of all trades?

This might seem obvious to those used to the WordPress way of doing things, but is it better to use dedicated themes to provide different functions, or to have all the functions on a single ‘jack of all trades’ site?

I’m finding that with WordPress, themes have a particular use-case they are designed for, for example a gallery, or buddypress site, or marketpress shop.

So if I run a multisite set-up with Pro Sites to add on additional functionality (e.g. level 1 is a basic blog and forum, level 2 adds on a shop) then I’d imagine it adding all the features the user wants to the same subsite. But is this a sub-optimal experience if say a dedicated shop theme is much better?

I guess the separate site route causes other problems though as the sites wouldn’t be linked together or look similar.

Any thoughts??