So you have to look at the game as a competition with the AI. I mean, it's much easier to steal a city from an opponent than build one from scratch, so if you have the military might and opportunity to do so, why wouldn't you?
It's kind of interesting you should say this. I Slash and Burn, EVERYTHING. Partly to force the AI back without leaving exposed cities in my wake (that they could theoretically reclaim with a mere Pioneer), partly because the 50% unrest penalty on taking a city from an enemy can be insanely prohibitive, especially if you haven't gotten your Fort cities up to level 4/5 yet and base unrest in your current cities is at, near, or above 50%.
On top of that, I find the AI tends to make fairly poor location choices (or at least, poor compared to a human strategist), which often leads me to want to re-settle their former lands anyway.
I'm at the tactical point where my city type decision tree is as follows:
If tile has 2+ essence, Conclave.
If tile has 4+ food, Fort.
If tile has 2 or 3 food, Town.
Ignore tiles with 1 food, unless grain is in reach.
Prioritize tiles with access to rivers for Forts and Conclaves, access to forests for Towns. Access to both is epic and trumps all other concerns except in the case of:
I need more mana/research production, ergo, prioritize Conclaves.
or
I need more unrest reduction, ergo, prioritize Forts.
In all cases, two lesser build sites are preferable to one awesome site, except where a 3 essence site can be garnered, which quite literally trumps ALL other concerns.