The only plugin I use is the xhtml validator. I don't want to rely on plugins because with upgrades come broken features. If you rely on something like that today it could be gone tomorrow and then you'll be frustrated with your own efficiency. I'll just avoid all of that right now and rely on myself and my trusty editplus.
Sure, but why do things the hard way? They don't automate anything for you, but they do help a lot. Almost like using syntax highlighting in your editor. I would be completely lost without my syntax highlighter. Sure, I can still code, but it makes it "easier" if I can have tags and attribute highlighted.
I would recommend you try them to see what they can do for you. It might ease your skepticism. I've used them for about a year, and I have never noticed any bugs. They are Firefox extensions, not plugins as such. They can be removed cleanly, and easily, and allow you to do more than just validate HTML.
Your skepticism is a bit like not using a car jack to fix a busted tire, or trying to unscrew a screw with a piece of wet paper, if you get my meaning. It's simply a tool to "help" get the job done. It does nothing for you (like WYSIWYG editors do for example), and you have complete control over every aspect.
You can edit your page on-the-fly, so no more making single changes, uploading them, then previewing them. You can see the style inheritance patterns. You know when you pull your hair out wondering why a style has been overridden? You don't have to anymore. You can monitor and debug Ajax requests (if you use Ajax). It can also help you see what someone's Web site is doing behind the scenes without scouring through obfuscated code. You can see the exact data sent to the server and received from the server.
Honestly, the list is endless. All the things it does are things that are extremely hard to do, or mostly impossible to do by hand without hacking your browser's code. These tools can save you hours of head scratching if used properly, and help you become more productive. (Note, I said said "help" you, not "make" you).
Anyway, I've had my say and I'm not one for pressuring people, I just hate the thought that someone as smart as you could be missing out on something so great. I am now happy that I have said everything I can to explain properly so the choice is yours to make. Please let me know the outcome.