I have a contrary opinion.
First of all, the Spider doesn't have a MacPherson Strut suspension; therefore, it's impossible to fit a strut brace. Back when I was doing road racing with my VW Golf, the class rules allowed strut braces. Since the Golf has MacPherson struts, the bar across the engine bay made a big improvement.
The Alfa Spider, on the other hand, has an A arm front suspension design. It's these A arms which carries the upper transverse suspension loads, completely unlike a MacPherson suspension design. The upper shock (damper) mount (in theory) only carries vertical damper loads, not transverse suspension loads. The mounting points for the upper A arm are located on a very strong part of the chassis.
These mounting points are a long way from the upper shock mounts, so connecting the upper shock mounts will not do much to stiffen the Spider suspension. You can't directly connect the two sides of the upper suspension mounting points on the Spider because the engine is in the way.
I admit to fitting such a bar to my Spider and it does look pretty cool in the engine bay; mine is polished aluminum. I couldn't detect any improvement in handling after fitting the bar. Having said that, since the Spider doesn't have a conventional top (roof) and the bonnet (hood) is huge, one might argue that any improvement in body stiffness, no matter how small, is a step in the right direction. Plus, once an owner has paid for an accessory, it's easier to say it's wonderful than to admit that it's a waste of money.
I have also fitted Eibach springs over standard dampers. They're stiffer and a little lower. The springs do indeed make a significant improvement in handling and they really don't cost much more than the so-called strut brace. I would strongly recommend Eibach springs, but I would not fit the shock mount brace if I had it to do over again.