I have several Pentax cameras and I have a bunch of lenses to go with.
I've got 500mm telephoto/zoom lenses, 28mm wide lenses and macro lenses but 90% of the time I use just the plain, old 50mm normal.

Zoom lenses can compress perspective in a way I don't like. Wide lenses can cause barrel distortion. How many times do I photograph something close up?

Also, don't forget that longer and/or more complex lenses lose more light. If I remember, my zoom lens is an ƒ-5.6. If I'm shooting 100 speed film I can end up in a situation where there is not enough light. (God forbid I use 50 speed!) Then, with a big lens, I often end up needing a tripod... more crap to carry around with me.

I'm not saying that these lenses are bad. I'm just pointing out that everything has its pros and cons.

Half the time, I don't even bother taking extra lenses with me unless I plan on using them. For instance if I'm going bird watching. In that case, a tele/zoom lens is indespensible.
The rest of the time, I prefer to travel light. I want to be able to grab and go.

Just use your 50mm and, as they say, learn to zoom with your feet.

Generally speaking, all other things equal, an ƒ-1.7 lens is better than an ƒ-2 but, as with everything there are pros and cons.
The faster then lens the shallower the depth of field gets at wide-open aperture.

With an ƒ-1.7 lens at a distance of 10 feet, your depth of field be less than 1 foot. At a distance of 5 feet, it might only be a couple of inches.
In a situation like that, you might have a little gambit to play: Open the lens up wider in order to get enough light to make the shot or stop down in order to get the depth of field.
You're going to be forced to recompose the shot so that soft focus doesn't matter or you're going to have to move around in order to take advantage of better light. (e.g. put a window at your back.)

If I had two lenses in front of me but I could only buy one, assuming they were comparable quality, I would probably take the faster one but I would do it with the understanding that wider ƒ-stops often have shallow depth of field.

Bottom line: Accessories and lenses are great but don't get too wrapped up in them. You'll be better off in the long run if you learn how to shoot the 50mm normal lens and when you are in a situation where you need them, the other lenses will just let you be better prepared.