Yes, but thousands of people will be using it live- because you built it and released it, regardless of the version number.
Surely there will be somebody out there creating upgrade files, even if it isn't officially advocated by the phpBB team.
"Raise your hand if you are going against the grain and running this thing live anyway...."
There are those that have a clue what they are doing (hint: people whom code and develop software for a living) whom are quite able to use whatever Dog-food™/Alpha/Beta code and run it in a (semi-)production environment. Such people are quite able to manually (either doing it by hand, literally or write their own scripts) upgrade between each milestone product.
Generally, such tasks are non-trivial, which by extension is also out of reach from an "advanced user" in the absence of a bundled upgrade script. Thus, such persons would be "stuck" with whatever version of software they have installed, not just exposing themselves to bugs, but also to possible security issues which may come to surface.
Case in point, phpBB3 Beta1 a very long time ago had the developmental version number of "phpBB2.1.0", which for all intents and purposed little different to phpBB2.0.4, the stable line of the time. However, there are still forums running "phpBB2.1.0", unsupported CVS code which has never been uodated and thus missed all the security fixes from 2.0.4 to 2.0.21 (or indeed any fixes if they've manually kept it up to date along the 2.1.x/3.0.x line).
It has also been stated very clearly that should there be a security issue with phpBB3 Beta1, whilst it will be fixed in CVS, we will not issue a new beta just because of this. Also in the absence of an upgrade script, this will also mean you'd have to wipe the forum and clean install from scratch.