How beneficial is speaking (or holding a workshop) at non-academic software development conferences (like PyCon, Strange Loop or LambdaConf) to future computer science career (applying to PhD and further)?
That really depends what you mean with "beneficial". One of my junior PhD students recently went (partially self-funded) to a relevant developer conference, and he felt he benefited greatly (probably much more than from an average scientific conference).
- He was able to establish great contacts to industry. This included potential partners for studies and interviews (lately a big part of our research), as well as potential internship opportunities. (as a sidenote, this mainly happened because this specific student is an excellent networker - you can't just go to a developer conference and hope that useful contacts will spring up automagically)
- He got a lot of perspective on how industry sees the current challenges of our field (as opposed to other researchers). This gave him interesting ideas for his PhD, and also tremendously boosted his motivation, as he saw that many of the things we are working on are actually quite interesting to big industrial players (such as, in our case, Facebook).
- He was able to give a presentation about our ideas, and they were perceived really well (much better than he expected, truth be told). This gave him presentation practice, forced him to collect and structure our early ideas, and further strengthened his commitment to the topic.
Now, since you specifically ask whether attending such a conference will be valuable for:
(applying to PhD and further)
I think you can safely ignore this aspect. In pretty much any career phase, nobody is going to read your CV and think "that guy attended this conference, he must be great". I don't even list attended conferences (of any type) on my CV anymore, simply because it seems so irrelevant to a recruiter.
@ff524 correctly notes that you were specifically asking about speaking at an industry conference, not just attending. I am not entirely sure about this, but I would assume that this may be a small positive factor for applications on a junior level (say master of PhD student), depending on the conference:
- Nowaydays, many of the industry conferences in my field are organized like un-conferences. That is, everybody can speak if they want to (maybe pending some light review for on-topicness et cetera). In that case, you are back to square one - speaking at a conference where everybody is allowed to speak does not count for much. It maybe speaks to your motivation, but really that seems like a very small and insignificant factor.
- If the conference is more like a traditional conference, where speakers are typically either invited or you need to apply in some way for the honour of speaking, presenting may be perceived as a sign towards your esteem, but this really requires that the reviewing professors know this specific industry conference, think highly of it, and know that speakers are competitively selected. I would not count on this to happen, but it surely won't hurt.
For any application after graduate student level, your research credentials are so much more important than any talks at industry conferences that you can safely ignore them for future applications.