Ahh, here we are, doing it again. Talking about the old and always present issue of backwards compatibility with shared/ duplicated software, because that is what this discussion can be boiled down to.
IMHO, a framework is supposed to lead by example. It must do things “as right as possible”, because it forms the foundation for your “modern” application. Does anyone disagree with that?
If you do, then you must also agree, supporting code still built to work for an old PHP version, like PHP5.2, is not doing things “as right as possible” and it certainly isn’t leading by example. In my mind, it is supporting and demonstrating… laziness. The wrong kind of laziness. Laziness to refactor the code to meet modern coding standards. It is simply wrong and as such, this alone is a clear reason why you shouldn’t build your site on Codeigniter, if you want a modern up-to-date application built on it. They aren’t being leaders, like they should be.
Now that being said, if you have no intention of also sharing your work (paid or free, a “one-off code base” as Anthony Ferrara called it), or you know it is simply a better fit to your or your team’s programming style, then it could be a great tool for you. You have to decide that. As others have pointed out, CI has been around for a while and is tried and tested, which means you can count on it. All that can also weigh in on a decision to use it and it would be perfectly fine to make the decision to use CI on that basis too.
Still, because they aren’t being the leaders they should be, IMHO, I’d still be wary of the decision to use it, because, one day they might just change things to finally break BC (when it is too late) and you’ll end up doing a lot of work, which you could have done up front, with possibly a little more effort and with a framework that is being a proper leader now.