Seven years, and my answer to 'do I need a security plugin' is the same.
What is a Security Plugin?
A security plugin is not a plugin like 'brute force protect' or 'limit login attempts.'
A security plugin is like Better WP Security or WordFence or a hundred other plugins that promise to scan your site and let you know what's changed.
This is not to say that first set of plugins aren't there to make you 'safer,' it's that those are single use, targeted plugins that address a single issue. Limiting login attempts prevents someone from trying the same attack over and over and over until they get in.
By contrast, all-in-one security plugins try to do everything. They scan your code, your data, and your site. They look for all the possible attack vectors and they try to plugin them.
What Makes That Secure?
That's the question I ask people. If a plugin adds in 2-factor authentication, I ask them what it does for them? Password expirations, captchas, file compares etc. Those are all good things, individually, but are they applicable for all people? What, specifically, about those things makes you more secure or not?
Now. Before you get all shirty with me, I am well aware of what all of those things are good for. With the exception of captchas (which are not accessibility friendly, please stop using them), all of those things make a lot of sense. You expire passwords and, one hopes, require strong passwords to make it harder to break in. But if you have a 2FA setup, do you need to require rotating passwords?
It's All About Thinking
Security plugins stop people from thinking about what's going on.
I've seen it time and again, people install a plugin that 'makes them safe,' follow the bare minimum of requirements, and then install whatever they want without thinking about it, leave registrations open, and oops, get hacked.
This is not to say that security plugins don't prevent some of that from happening, but they're often an 'after the fact' solution. That is, usually a security plugin doesn't know to block X until X has been exploited. That's kind of the nature of the beast, though, and why WordPress and many other CMS developers don't release full details on security fixes until they've been out there for a while. They want to give people a chance to upgrade before saying "Hey, y'all who didn't are super vulnerable."
It's Also About Speed
Security plugins also have a tendency to make your site slower. This usually comes up when people have turned on everything that comes with a security plugin. Which goes right back to my point about thinking. The user doesn't think, because they're not yet educated, about the impact of the code on their site.
To put it simply, the more things you ask WordPress to do before it can load a page, the slower it will be to load a page.
Pretty cut and dried, right?
What's My Answer?
I don't call this the 'right' answer or even the best one. Not everyone has access to my resources after all, so it's not fair to say "Hire Mika to think for you!" But to me, the best answer is to use the resources you have intelligently.
Firewalls, from a server side, are all but a requirement to me. If your web host doesn't have one, and most at least have ModSecurity, get a new web host. If you disabled it on your site because a random plugin doesn't work with it, delete the plugin and turn it back on. If you can't move to a new host, look into firewalls like Incapsula or Sucuri. Put something between users and data.
Site Scanning is a great tool, but don't run it on WordPress. A great example of smart security scanning is VaultPress. It's a remote service that has a copy of all your files and it scans the copy, not your site, for issues. There are other services you can use that scan your site without affecting traffic. Again, web hosts often have tools for this.
Be Aware of what's going on. Don't just let security be a black box. Make sure you know what kinds of attacks are common on your site. If you're hit by a DDoS, for example, where they're just hammering your site to take it down, a 2FA plugin will not help. If they're trying to log in all the time, a scanner is probably not what you need.
Lock It Down. If you don't need it, don't use it. If you don't need it on, turn it off. Update regularly. Don't install everything under the sun.
Don't Buy Into FUD
This is a tricky balance. On the one hand, I want to say 'don't panic if your favourite security plugin of choice tells you everything doomed!' Remember, they're trying to sell you things. But on the other … don't think everything's fine and dandy.
It's not a simple solution. You have to simultaneously be aware of problems and not overwhelmed by them. You have to learn how to care about which ones are important to you and which are not.
In a word, you have to think.
And there is no plugin on the planet that can think for you.