In our area several agencies switched to DPL codes, especially on VHF channels, to avoid long distance interference. We swapped our VHF channel to a DPL code since most of the PL codes were used by agencies that could be heard during temperature inversions, early mornings, or other times when VHF conditions were up.
With narrowbanding you also have to contend with operations closer in but on adjacent frequencies, increasing the need for more codes to mask other users. With only 38 standard and a dozen not-quite-as-standard PL tones, the additional DPL codes give you a lot more options.
Current radios are easily programmed for both PL and DPL codes, older radios were not.