In my tests, the G31 A (Z-only) calibration worked significantly worse with an offset probe than with a center probe. It was bad enough to ruin first-layer adhesion (lifting in one place and dropping enough to block the nozzle in another), and that's why it's disabled. Heuristic calibration was also worse by about 40-50 microns. Since the physical system already has misalignments, adding the requirement to make the effector go to an XY position other than where the probe is, makes things worse. It introduces additional error into the system, and that error makes it harder for the correction to do its job.
I understand about it being more convenient to use a side-mount probe. I used one for many months, and I liked it at the time. My Hall-O probe mount on Thingiverse still has side-mount provisions. However, at the end of the day, it's better to use a center-mount probe. More effort to do it that way? Yeah, but there's a huge tradeoff involved. Unless the printer is moved or harshly bumped into, there should be no reason to recalibrate it. In exchange for messing with a screwdriver and a wrench for a few minutes, you get months/years of printing that's more positionally accurate than you can get with a side-mount probe. The more accurate your printer is, the easier it is to print other people's stuff, and to design your own stuff without having to add special provisions that are specific to whatever way your own printer is messed up.
These days, with a microswitch-based Z-probe that costs well under $5, I'm designing my parts with ZERO dimensional error tolerance. If I make a 20mm square hole, and a 20mm object to sit in it, they will fit perfectly. That's so much better than printing something, having it not fit because the printer's calibration is off, having to massage it to be just right, then it doesn't print right on someone else's printer, etc. Trust me, the few minutes you spend messing with a center-mount probe will pay off big time!