I seriously am getting a weight-lifting kind of satisfaction out of my fretting hand gaining strength and agility with practice. I know some people need to start playing tunes right away to stay interested, or so many teachers think, but if I just zen out, playing varying patterns up and down the neck and hearing the notes get fuller and richer as my fingers gain independence is quite satisfying in itself. Odd also--mistakes or variations sometimes reveal techniques to me, rhythm patterns and such. Tired fingers will accidentally do a cool hammer-on pattern or legato or whatever. I can grab that mistake, repeat it 100 times and its mine whenever I want it. Programmers talk about a larval stage, a period of intense single-minded activity that, once passed through, will produce a programmer. I think this is somewhat the same. Am I making any sense?
If you really wanna get your left hand into shape, consider picking up the upright bass. After playing upright for a few years, when i pick up an electric i feel like i can crush the neck with my hand strength. As for your other points, i totally hear what you're sayin!
I just started out playing bass a few weeks ago and I tried learning songs right away. I realized that although it's fun to try to learn songs, building up agility is more important to me right now. It may not be as fun at the moment but I know it will pay off more in the future.
It sounds like you’re gaining endurance, not strength, which is much more important than strength. Unless you intend to use high action, very little strength is needed for playing electric bass, making most of the hand strengthening tools and toys by and large unnecessary. Your pinky usually needs some work early on, but that’s about it. Then again, I wasn’t brought into existence by a conspiracy of birds and wind and fire, so your situation may be different.