I cannot speak on a general rule or anything, but many older physicians (not so old to be grandfathered into a board) who are IM subspecialized end up not taking the IM boards after a while. I know plenty of PCCM physicians who stopped IM and just maintain pulmonary and CCM. the same goes for plenty of other subspecialists. Many of the hospital based general cardiologists at one large tertiary center I go to (part of the faculty and all that) are also just boarded in cardiology
Personally, I plan on keeping my IM boards because I do a little primary care on the side. While it is true you do not need an IM board to practice primary care as a specialist (after all knowledge is knowledge), one would need an IM board if one wanted to be listed as potential PCP for insurances. This is only relevant for if you plan to open your own PP. If you are an employed PP physician, then your employer will lay out the terms of what you need or do not need.
I know a few PP general cardiologists who also advertise as PCP (he/she owns the practice). They would need the insurance companies to list them as PCP and specialist. That would require the IM board certification. While hospitalists can work as board eligible, the IPAs that negotiate with the insurance companies for the insurance coverage usually requires IM board certification. While they cannot "double dip" a patient, this does allow these physicians to expand their patient base without the need for relying on referrals. After all I'm sure most older metabolic syndrome patients have some reason to get an echo, stress test, holter, carotid, and nuke... plus patient seem to like it when they brag "my PCP is also my cardiologist!" (but the same does not hold when the PCP is another subspecialist... cardiologists get all the glory and renown)
If you plan on becoming super specialized as Interventional Cards, EP, structural, CHF, etc... there may not be any real reason to keep your IM boards. I suppose if you planned to cover a lot of CCU and want to be the "best internist" you can, consider the IM boards. But you just consult whatever subspecialist you need anyway. If you plan on being general cardiology, consider keeping IM boards for the future when you decide "i've had enough of these employers taking half of my productivity and feeding me RVUs, I am going to open my own practice!"