Why do IFR charts show TACAN paired frequencies when there is no collocated VOR?
Your supposition is correct. Tuning the paired frequency will allow you to receive DME information from the TACAN.
If your DME tuning is through your VOR/ILS receiver, the VOR output will be flagged no computed data (NCD) as there's no VOR signal to receive.
The DME will tune the appropriate paired frequency and the DME should display normal data.
Having DME data alone is not all that helpful. However, if you have an FMS and a multi-channel DME, the FMS will tune the DMEs independent of (or in conjunction with) the VORs. This will allow the FMS to perform DME/DME navigation. That was the primary FMS navigation method until GPS came along. These days, DME/DME is a backup to GPS or GPS/IRS navigation.