You don’t have to have an aspect be two-sided, but if it isn’t, you should balance it with a one-sided the other direction.
For example, there’s really no negative to my being very agile and dexterous (i.e. Graceful as a mountain goat). However, there’s also no real postive to having abandonment issues. So although two-sided aspects are nice, you don’t have to make all your aspects two-sided as long as you basically have an equal number of boons and banes.
And just about any object/person aspect becomes two-sided if it’s something that can be left behind or forced. For example, if I’m in a situation where my car would be useful, the GM can force it to be in the shop, or to break down, or something similar. I am now forced to be deprived of an aspect, so I get fate points for it (or pay fate points to guarantee it’s still there). Same thing with the belt. If my belt is taken from me during capture, and I run into a situation later where I could use it, I get fate points for having to do without it. (Say for example, I’m locked up in a holding cell. Now, I get no fate points for not having the belt for escaping the cell, since naturally prisoners have all their stuff confiscated, and no captor is going to leave me my lockpicks. However, once I escape the cell, if I’m not given opportunity to recover the belt and tools, I am now forced deprived of them, which would earn me fate points because my aspect is nerfed.)
Mr. Z acts as both a boon and a bane as well. I can use him to help with getting missions and fencing goods, but he can also manipulate me into taking missions on that the DM wants.
Basically if the DM is controlling your aspect, you get fate points. If you are controlling your aspect, you pay fate points.