EDNS in DNS clients

Hello,=20

I'm writing a C++ module that provides simple DNS lookups - for now,
only SRV and A queries, with AAAA queries planned for the near future.
I'm using libdns from BIND 9 for message rendering/parsing, but the
sending/reply notification I'm handling myself (asynchronously, of
course, otherwise getaddrinfo() would've been enough). I'm expecting
this to be used to query some caching nameserver that would do the
hard part of the query (following referrals, for example). Now, my
question is: how important is for this kind of module to support
EDNS(0)? How common are this kind of packets in non-DNSSEC
environments? I suppose it depends on particular setups, too, but I'm
looking for something more specific than "your mileage may vary" or
"it depends" :).

I know this question isn't very tighly related to BIND 9, but I don't
know of any better place to ask it.