Hi
I have a single library for all the parts I use in electronic layouts and another that is just for mechanical layouts via the board editor.
When I first started, I made separate DEVICES for different kinds of caps, say, when in retrospect I only needed to have one polarised cap device and one nonpolarised cap device, just so the schematic symbols would be appropriate, and each with whatever number of packages I have devised over the years. This would certainly make the library smaller
over time I have come to use only two of the cap devices in my library as described. I cannot delete the rest because Eagle considers them to be "in use" by whatever projects still use them.
Another example is resistors. The different power ratings have different sizes of course, but why make a different device for each power rating? Just make all the packages you need and then it is easy to drop resistors on the schematic and change specific location packages as required. Even with the same wattage resistor, you sometimes need longer or shorter leads, so you might have 50mil or 100mil increments of lead length and thus several "half-watt" packages just different pad spacings and maybe modified silk.
The mechanical library is for templates and outlines of things I commonly use. I design my chassis using board files, so have elevation outlines of pots, jacks, switches and things that show the PCB alignment and below-board clearance needed for solder pins.I have outlines for brackets and things that bolt to boards for ground bonding and other stuff like that.
I think it would be time consuming to have the electronic components separated into separate libraries - that is Eagle's default. CMOS chips in one library; BJTs in another; caps in another, resistors in another - it saves some scrolling at least to have what you use most often be in one place. At least for me that works