Hello and thanks for your feedback! While most sites using DT Register would certainly not require 256M on the memory_limit, this is not anything I'd be concerned about. On larger, more active sites, a higher limit may very well be needed. Still, 256M is not really a high limit. It's becoming more normal these days actually.
Our test sites all have 64M on the memory_limit and rarely run into a memory issue. It just depends on what all you are doing... how many fields... how many events... how many users... and of course, what else is going on with the rest of your site. Plus, your hosting setup plays a big role too.
On another note, we've also seen scenarios where people using a newer version of PHP that isn't as well tested can sometimes APPEAR to have memory issues.
There are just too many variables that can affect your overall memory_limit need. We know DT Register is a large extension and certainly will require more than running a simple form builder extension that does nothing but spit out form input fields.
Over the years of development, we've occasionally made adjustments to reduce resource usage. This is something we always keep in mind and... when we see an opportunity to improve it... we do it.
In the meantime, in our opinion, a 256M memory_limit isn't anything to lose sleep over. At all. Thanks again!