...Say you have a UHF low split radio (useless!) with a desirable feature set,
I wouldn't say totally useless. They are great for monitoring FED/Mil traffic, and don't forget the 420-430 MHz Amateur Band. I, like a number of other hams started experimenting with FM in the bottom-end of 420 MHz, and put up [coordinated] repeaters. At the time, I thought it was a new idea, only to find out that there is a 420 repeater system in the Dallas-Fort Worth area that dates back to the 1970's that is still active!
I put up a local repeater in Lynchburg (VA) in 2001 on 420.050 out, 426.050 in (136.5 Hz tx & rx). In 2007 it was upgraded the Mastr3 to support P-25 (adding a DSP card to the shelf and reprogramming). Dual-mode, it supports both FM and P25. (NAC $293 tx & rx) There are other 420 repeaters, in Los Angeles (both P25) Dayton each year has several show up for the Hamvention, Washington DC, Raleigh, Miami, and elsewhere. Hams have started to discover this under-used segment of the band, and have been utilizing it in areas were ATV is not present.
[Tests have demonstrated time and again that 5 kHz FM voice on 420 MHz can coexist with ATV, with no interference or harm to either mode !]
If you do a lot of Public Service Events, 420 has shown to be a handy place to "hide" from listeners,
especially the MEDIA! Command Nets freely operate, as listeners see UHF radios, but never think to look outside of the 'normal' 440-450 MHz frequency range.
One nice thing about the P5100/P7100 family of radios is that if you know what you're doing, you can transfer the feature set from a radio to another. Say you have a UHF low split radio (useless!) with a desirable feature set, and a VHF radio you only WISH had those features. Well, the features CAN be swapped between the two radios but it requires someone who is adept with a soldering iron and also knows how to use the radio maintenance tools. The ESN chips need to be swapped and the feature strings, too.
The practice of swapping ESN chips was the reason Harris changed how they serialize the radios starting with the M7200/P7200 product line around 2005. The ESN and Model ID code (portable system/scan/select tier) can be changed electronically, but the procedure is a closely held trade secret.
When the company discovered that Service Techs were swapping ESN chips when repairing radios, rather than upgrading the ESN of the radio's replacement Control/Logic Board,
they were not amused.
On top of that, the practice of ESN swapping for mere upgrade was becoming known. (Hence the number of Aegis & ProVoice 35-50 radios on six meters and two meters! Especially among the Prepper folks.)
The Orion, M-RK, J700p, J725m, M7100 and P7100 all share the same exact ESN chip, their Feature Encryption structure is identical, and can therefore be freely swapped between radios.
My humble 2¢ worth....