Knights can be worthwhile to get in Monarch (and possibly Emperor - needs more testing).
In many cases, the optimum path for expansion goes to a tech lead towards Cuirassers and Rifling, but that is not always the case. Sometimes, it beneficial to attack a Civ now rather than later, even at tech parity. This is true of teching Civs with low unitbuild prob - particularly Mansa. If you can attack him at tech parity, you get good odds for a nice return on the hammers.
I do not wait for Guilds to attack a late medieval war. As I mention in other threads, my personal defining units for a late medieval war are Trebuchets and Pikemen - both available at Engineering, which is THE military tech to get for that kind of a war.
However, I do stake a look out for the possibility of teching or trading for Guilds and Feudalism easily, though neither are strictly necessary for teching upwards to Cuirassers (IIRC - could be mistaken).
Guilds provides your Workshops with +1 hammer output. With Caste in play, that makes Workshop grasslands and plains better hammer output than the equivalent hills - and that time frame is usually just at the time when Slavery output is thinning due to larger city sizes - just the right time to want Workshops. Feudalism, of course, is desirable for the Longbows - essential for defending your positions in a war or a backstab.
Going forward, teching Guilds during wartime encourages you to tech up Gunpowder, Chemistry (for even more Workshop hammers) and Steel after that (for Cannon). Compared to peacetime teching to MT for the Cuirassers, it's significantly slower, but what it does allow you to do is to acquire land through war while having a significant and powerful army - with upgrades to your military arsenal imminently forthcoming. Knights and better Workshops are a great boon for any military endeavor at the time (provided you already have Engineering).
Each of those techs - Guilds, Gunpowder, Chemistry, and Steel provides you with some sort of useful military unit, an economic advantage for production, or both. Given that the Lib path is techable through bulbing, you can tech through Paper and Education largely through bulbing while teching the lower path techs at the same time.
Obviously, putting forth all effort to path up MT gets you there faster, but you have to weigh the speed of that teching against the benefits of having Knights now rather than Cuirassers later. Sometimes, it's good, and sometimes, it's not so good.