Do you think dedicated leads matter anymore?

A dedicated lead is essentially a Pokemon that is geared towards being your lead most, if not all, of the time. With the advent of team preview, dedicated leads such as suicide leads are no longer common. (The most common form of suicide lead was a fast Pokemon with an entry hazard move, Focus Sash, and maybe Taunt. The idea was to guarantee the set-up of entry hazards while attempting to prevent your opponent from doing so.)

Do you think they are a waste of space on a team? Do you have any ideas on what they could be used for, if they're worth a team slot?

They are a waste imo. Using all 6 of your pokemon is basically required (and I always found leads weird in the first place). Team preview didn't help the idea of leads any either.

They do seem like a waste of space in your team.
My lead Pokemon for competitive battles does badly poison and seed the opposing Pokemon, though.

I feel there is no need for a dedicated lead now, just supports that carry SR. You could use Mew to setup SR and checks threats/BP boosts to other members or have it serve as a Wish passer. Team Preview has basically killed that, admittedly I've been trying a dedicated lead right now and it's working quite nicely.