I almost always pick the same 3-4 servers when I play, and 3 of those servers are almost always empty when I join. I play the bots for a bit, then the usual crowd shows up. By the time the map changes, there's usually at least 15 or so players on the server and most of the bots are gone.
If I see a server with just 1-2 players and a bunch of bots, I'm usually inclined to join.
Why?
Not because I want to bot farm, because as I said, I already have everything maxed out.
I join in order to help the population and eventually gain more people.
If by chance they are trying to bot farm and don't really want to play the game, then I come in and ruin their day.... win win for me. When I join these servers, I specifically pick the side that has the least amount of real players and I specifically target the human players while I kill bots in between waiting for them to spawn back.
Everybody has their own interests in playing the game. There are plenty of people joining the heavily populated servers, so they don't need me.... I join the lower populated servers to make things interesting.
Even if a server only gains a total of 5-6 real players and the rest are bots, that's still more than enough players to make the game interesting and a challenge.
Bots and Human players have their own little quirks when playing against them.
When you're playing nothing but humans, you have a certain frame of mind set for combating them. Humans will usually try not to do the same thing twice if it kills them and will often change classes to get the upper hand, but there are certain traits to human players that you can predict what they will try and do next and drop them before they can get the upperhand.
When you're playing nothing but bots, you have a different frame of mind and with their bot reflexes of head shotting you before they even turn around, you need different tactics. They can do stupid and very predictable things, but they can also drop you faster than a human could and they can melee insta-kill you before you have a chance to know what's going on.
However, when you have a server mixed with real players and bots, things get really interesting because you then have to use both the above frames of mind at the same time and that can be very difficult. You can be targeting a bunch of bots and suddenly the humans pick you off because you stayed in one spot for far too long and made too much nois. If you target the humans and find a nice spot that would typically take a human a long time to figure out, you can easily be picked off by a hipshooting bot from halfway across the map. The humans can distract you from the bots coming at you, and the bots can distract you from the humans flanking your position.