Not using an iButton results in a codeplug version left not updated so you get "new" firmware with an "old" codeplug version and features in the firmware may not work.
Unless they are being used on trunking systems (legitimately not whacker crap), or the HOST/DSP is woefully out of date (like below 12.xx), no reason to go through the headache. FWIW the only real improvements to the DSP started at 12.xx which does give better P25 audio but honestly, why bother if it works, don't fix it?
The process of firmware upgrading can be risky unless one is using genuine OEM cables, iButton, etc. The XTLs are cranky radios and sometimes one has to devote 2-3 hours to update a single radio if it fails, is way out of date (requires incremental bumps) etc. I've done hundreds of them in my career and even with the right stuff, ended up with around 10-15 unrecoverable bricks.