The question is not what percentage of hosts currently offer version X, but rather what percentage of hosts are unable to offer version X.
A big issue is what version ships with the distribution that hosts use. LAMP stack pieces must be compiled together, otherwise they won't work. Therefore if we require hosts to compile php in order to use phpbb this is a significant, and likely unacceptable, burden, as they would have to also recompile everything else running in their apaches.
The current situation is all major distributions are shipping 5.3. This means their QA processes verified that their php packages work with 5.3. 5.3 has been around long enough and is installed on enough servers that all maintained php software by now should be 5.3-compatible. Which means there should not be a problem with using 5.3 in production at this time.
What that means in turn is the only reason hosts still run 5.2 is because they have no incentive to upgrade to 5.3. With ajax being added and stylesheet refactoring we have a choice between reimplementing assetic, not shipping asset management or requiring 5.3 to use assetic. Of these three options requiring 5.3 has no drawbacks other than requiring hosts to upgrade, but this is exactly the sort of incentive they need to upgrade.
Erik Frèrejean wrote: risk a big portion of the user base not able to upgrade
This risk is essentially nonexistent.