You can write good clean, maintainable code just as fast as anybody without an IDE. The problem is, not everybody else can or is willing to learn how. The majority of the Java and C# I've encountered is completely illegible without an IDE and is written more like procedural code that happens to have class syntax wrapped around all the methods. I haven't seen as much PHP but from what I have seen there, you're probably going to run into similar scenarios when working with other devs of mixed quality (or on their legacy code long after they've quit/been fired).
We don't like dependencies. I just lost a whole day to trying to understand where this Java build is going wrong between Eclipse, Maven, and Tomcat. The last thing I need when I have to write code for umpteen browsers that don't always agree on what my code actually means, is my editor getting its fingers in the actual running of the app with its own set of config files and plug-ins and other completely unnecessary crap that should have just stayed at the command line where keeping it simple is the only real choice you have.
We don't need them for debug. Run the app? Hit refresh. Debug? Load the page up in Chrome (or use webkit for node), right click, inspect-element, look at all those tabs. On the web, our debug tools are built into the environment our code executes in and everything is so plastic we can modify code in the console just to see what impact changes have. In something like Node, everything tends to be so modularized we don't have to link 10 million dependencies to our work just to see if it works and do a little debug with logs and the like and we can still pull webkit or other tools into the mix for debugging our code.
No static types. The idea that static types protect you from something other than learning how not to write code that makes you paranoid of your own data flow is one I try to challenge almost daily on Stack because I come here to blow off steam after, you guessed it, looking at some truly awful Java and C#. With strong OOP that actually leverages encapsulation, you're not packaging/repacking, validating and revalidating at 50 steps in a gigantic chain of complete BS. You're checking contracts at the door and then working with flexible data structures that don't get in your way. So no, I don't need to hover over the vars in my code to see what types they are because that should be pretty damn obvious in a language that has like 5 core ones if you're naming things well, keeping your concerns separated, and not over-complicating things by implementing practices/patterns originating from C++ roots that don't make any sense even in C# or Java because Extreme Programming or some other standards-fabricating/advocating industry said so and people want that !@#$ on their resume.
It takes a fewer devs to handle the same level complexity. Fewer devs means fewer people reinventing perfectly serviceable code, implementing Kool Aid d'jour and getting away with it, etc.
IMO, yes, professionally speaking you will want to learn your way around a popular IDE if you're working with PHP primarily. I get the sense there is something of a mixed vote on that topic from PHP devs but also just knowing a thing or two about how to debug and navigate a nasty PHP codebase (check out the PHP that lives under Drupal some time) could make you invaluable to a team of Notepadders when the time comes.
The best advice I can give on learning an IDE is to find something really big and nasty written in PHP and do some debug with an IDE to learn your way around it. To find something like that, look for code that was written by an excessively large group of devs, none of whom ever used anything but an IDE.