I've been reading Harrington on Cash Games vol 1 and vol 2 and he usually uses a mixed strategy for specific decisions

e.g. raise 80% of the time, call 20% of the time, or even 3 way mixed strategies pre, e.g. with JTs in early position, fold 20%, call 40%, raise 40% or something like that. And he also advocates using the second hand on your watch as the RNG to determine what you do.

I very rarely see comments from people here giving a mixed strategy and I wonder why that is,

1) Is it we just prefer to be definitive?
2) We don't think mixed strategies have much value?
3) We usually know much more so we can be more definitive?
4) We don't trust Harrington

etc.

It's been a while since I've read HOC but from what I recall, he uses that method when he's playing against the same player pool. If you are in the same boat, it's a fine way to go. If, like me, you rarely play the same players, it's not necessary. Go for the line that will provide the highest EV.

Mixing implies balance, and balancing is simply shooting self in the foot in LLSNL.

Players in these games are dreadful hand readers, so sacrificing in-the-vacuum EV to disguise your hand is a poor investment.

Even if our opponents get the general idea that we play draws aggressively or always raise AA preflop or the like, there will be exceptions that come up in the natural course of the game, and I don't put faith in villains to piece together that we might actually have AA in this limped pot because there were several LAGs acting behind us against whom we had a dirty l/rr'ing range or that there are many spots where we might flat with the NFD, etc. In this way, we will end up only raising with our draws like 60% of the time, calling 25% and folding 15% (just pulling numbers out me bum), and only raise AA like 99% of the time, etc, and we don't need to actually stray from the optimal play to get to these percentages.

Even if you were playing in tough, mid-stakes online games, mixed strategies with any one hand in any one spot would be like 5th on the list of ways to vary your ranges.

We usually prefer to be exploitative, so mixed strategies take a backseat to finding the most exploitative line. Your strategy should be naturally mixed just because you will find yourself playing under different table conditions where the best strategy differs. I've played in a room where one table requires calling a 10BB raise just to see a flop and the table next to it is a massive limp-fest at the same stakes. Sometimes, the limp-y table is the more profitable.

Some people are strongly biased towards aggression. A mixed strategy involves mixing aggressive and passive play with the same hand, not just mixing strong and not-so-strong hands with the same level of aggression. Some posters just aren't wired to shift into a more passive gear.

Harrington is talking about games where most of your opponents are actually good poker players. This is rare at low stakes and if you run into a table like that you should be looking to change tables. Also Harrington is talking about playing the same pool of opponents many times. Even when your opponents are bad you need to mix up your play a little when facing the same group over and over. When you get to the section at the end about playing weak games he throws most of the balance concerns out and concentrates on straight forward value betting.

His concern with balance also seems bigger then it is because so much of the book is about preflop opening and the first flop bet. These are the two situations where balance is the most important. The book itself points out that balance is much less important on the turn and river.

Read the section on weak games. You'll see that Harrington advises in low level games to skip balancing and just go for value. He states that 1/2 is a weak game.