You use English within a variety of discourse communities, each with its own standards and folkways. In some of those communities—US teenagers, for instance—any use of relative pronouns, much less use of relative pronouns as the object of prepositions, is indeed likely to mark you as "bookish".
In other communities, however—students of formal logic, for instance—such uses are not only unremarkable but positively desirable: they mark you as "educated" and your discourse as "considered".
So the question is really How do you wish to be perceived by each of those communities? If it is important to you to appear hip among the young, you will cultivate their dialect, and eschew relative pronouns; if you wish to be well-regarded among scholars, you will cultivate their dialect, and embrace relative pronouns, taking care to place them after the prepositions of which they are the objects.
You may of course cultivate multiple dialects, for all the communities you work with; but that's going to take a lot of your time and energy. It is really much easier to decide who you actually are (or want to be), and construct just one idiolect which reflects that identity. Your energies may thus be better spent communicating your ideas intelligibly; and anybody you want to associate with will accept you for who you are.