There are advantages to this and of course, disadvantages.
Lifting slow gives you muscle mass. Lifting fast gives you muscle power.
Now what I mean by power is the ability to exert the same amount of force at high speed, delivering power. This is what you want in sports and in all reality, in real life.
Lifting slow really only trains your muscles at that speed. They cannot generally apply that force at a higher speed simply because they never have. Your muscles gain mass, yes, but they don't get "power" per-say.
This is something that is extensively studied and applied in professional sports - they all train "explosively" for a reason - when they need that strength, they need it NOW. This is something I've always been taught and still learn about every time I go workout with my team's trainers. Speed and form are what are important to develop "usefull" strength.
As one of my coaches once said, he'd rather take a punch from a juiced-up gym monkey than from a powerlifter or an athlete. Why? Simple physics pretty much. Momentum = mass x speed. Higher speeds that the contact is made at will have more momentum and hurt a heck of a lot more.
I personally do not lift slow. It has no application to what I do. I never will move my arm at painstakingly slow speeds. I will, however, move my arm at high speeds playing sports. Heck, in everyday application I will move my arm at a higher speed than what "slow-lifters" do in the gym. Why train at a speed I will never use.
And as to the ego thing - pretty much wrong. Those of us with proper form and explosiveness do not load up the plates to stoke our ego. We load them up to get a full workout and to tire out our arms. I work my arms to exhaustion with explosive reps whenever I bench. And I see improvements rapidly - I get stronger, more stable, and more solid.
The "slow-lifters" are doing it to tear more muscle during the exercise so that when it heals, it gets larger than it would. Now that would be stoking the ego in my opinion.
(I'm going to come back to this tomorrow and make sure I made sense writing it. I think I did... but hey... it's 5am here.

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