I think it is very helpful but not critical.
Software will usually get you in the ballpark, and if you are okay with ballpark, you probably don't need it. I have been using one for years, and have found that in almost every batch, I was within plus or minus 0.1 pH of my target from the software (almost always slightly below target), so I consider that a pretty good validation of the SW.
I tend to brew styles that are all in similar families (German lagers or pale ales), so there isn't that much variation in the ingredients I choose, so I expect a degree of consistency. SOme people might think that being off from your target by 0.1 pH is a big deal. There might be cases where you are aiming for a very specific effect where more accuracy matters more, but in almost all cases, if you aim for the middle of the optimal ranges, you won't be far off, and I can pretty much guarantee no brewer ever can tell the difference in a beer that was mashed at 5.3 instead of 5.4.
All that said, I still use one to continue to validate my process and ensure I'm not drifting more than about 0.1 from my target.
Also, I have found times when it was super helpful to have it though to correct errors. For instance, recently I was doing two batches. I did not realize until I was in the mash that I had added all of the acidulated malt for BOTH batches to one. Panicked, I checked the pH. instead of the 5.5 I was aiming for, I was at 5.1. Okay, not ideal, but not catastrophic either. I was able to correct it by not adding any acidifying additives (CaCl, gypsum, etc) to my infusion or sparge water, and by the end of the mash, I was up to 5.3. Not ideal for me, but good enough. Without a pH meter, I might have just scrapped the batch or not known how well my corrections had worked. Information is power.
YMMV