I agree that dedicated leads are generally poor decisions for Pokemon on a team. Obviously this varies with Pokemon, playstyles, and even players, but generally they are fairly easily exploitable. However, this doesn't mean that Pokemon that are often seen as dedicated leads are bad ones to use. Most Taunt users are still good Pokemon in their own regard, and Taunt is just a great move in general. You can also use Pokemon as leads quite often without that Pokemon being a "dedicated lead." U-turn and Volt Switch users, both fast and frail like Genesect and slow and bulky like Rotom, are good examples of Pokemon that work well in the lead position yet can still function very well outside of it.
There are, of course, also other ways of using Pokemon that are typically considered as suicide leads. I ran into a really crafty user of a suicide Sticky Web Smeargle who adapted really well to Defog and common spinners by waiting to bring it out until after my Excadrill had already been taken out and my hazards had been cleared with his own Defog user. Seeing a Mandibuzz and a Smeargle on the same team led me to believe that he was using SmashPass Smeargle or some gimmicky set, so I didn't mind losing my spinner so much towards the middle of the battle. I let my guard down pretty early on, and it came back to bite me. Had I not have had Sub Kyurem-B, which pretty much steamrolled through his team, I'd have probably lost that battle.
What I'm getting at here is that more often than not, better players are malleable in how they use their leads. Lead Deoxys-S can still work, but it requires a much more precise team in order to function reliably. You're almost always better off by taking time out to assess the opponent's team as soon as Team Preview is revealed and basing your decision on a) their most likely lead or b) what threatens the opponent's team the most from the beginning without giving up too much momentum. Pokemon is ultimately a huge mind game, and locking yourself into only one possible lead is extremely counter-intuitive.