Thanks for your feedback.
I wrote the plugin in PHP 5.3 quite simply because that's what I use. Everything I write in PHP - with very few exceptions - is for version 5.3. I am fond of the new features, particularly namespaces, which give us a much more elegant way of organising code - naturally - so there's no good reason for me to use anything else.
I wrote Portico to help me out on a personal project - again, based on PHP 5.3 - and open-sourced it because I thought there may be others who would find it useful.
I agree that I may be limiting my audience, but working with an older version of PHP from day-one, when 5.3 has been gold for 18 months or so, doesn't make a lot of sense to me - and that's discounting the fact that I like using the features of PHP 5.3. I could well be wrong, but I think now is the time people start looking more closely at PHP 5.3, because hosts are starting to roll-out PHP 5.3 to users of shared hosting - which should rapidly increase its use.
That said, however, if I get a lot of requests from prospective users, developers working with 5.2, then maybe I'll consider down-grading the code. (That still sounds a bit bonkers to me, though: rather than upgrading to PHP 5.3, we down-grade code. Anyways.)
Perhaps you could publish a patch for PHP 5.2 users, since you've done all the work.