Hi we have been using MC for a while, but not connected to SFDC. We are currently use email as subscribers key.
I understand that tracking info will not come to SFDC if we don't use Contact/Lead ID as subscriber key. If we keep using email as subscriber key, are there any merits to connect 2 clouds?
Not really, as Marketing Cloud Connect will only recognize subscribers that use the Contact/Lead ID as actual subscribers. This means any cross-cloud functionality you normally would be able to use isn't available in your scenario.
Though you could use it just to exchange data between clouds, like automating some report-imports to Marketing Cloud and then using SQL queries to populate a new Data Extension that sets the Email attribute as the primary key.
With custom API events you might get some standard features to work, like synchronizing communication preferences by using Microsites or Cloud Pages.
Maybe someone has built a custom SF object-mapping with Data Designer with Email Address as the ContactKey and been able to use it with Journey Builder, but I somehow doubt it..
Anyway, just because you currently use Email Address as Subscriber Key in Marketing Cloud doesn't mean it will always be / have to be like this. Since you are using both Clouds, I would advise checking what else comes with Marketing Cloud Connect.