Of COURSE a bass or guitar is a tool - for creating music. And it's a tool that we as musicians need to depend on.
But - a real craftsman takes care of his tools. He treats them with respect, does not use them for purposes they are not designed for. Example: he doesn't use a Crescent wrench as a hammer, he gets a hammer of the correct weight for what he's doing. He doesn't use a screwdriver as a chisel, he gets a chisel.
And his tools are not left out in the rain, or put away covered with dirt or corrosive materials; they are cleaned before storage. They are maintained in the best possible condition. Screwdriver ends are ground to correct fit; chisels are kept sharp; wrenches are wiped clean and perhaps oiled lightly at times to keep them from rusting. There is good reason for this: when you depend on your tools performing as they were designed to, then you know you must keep them in good condition so they won't let you down.
In contrast, when I see tools that are beat up, covered with junk and obviously not cleaned or well maintained, I know the owner is careless and doesn't take care of the tools he depends on.
The same goes for basses and guitars. When I see one that has been well maintained and cared for, I make the assumption that the owner cares about the instrument and knows how to care for his/her instrument. It shows in the instrument's condition.
When I see one with scars, stickers, burns and other damage, I figure the owner either doesn't give a d**n about the instrument or is one of those careless people who never learned to take care of their things, and who probably never will.
And my opinion and impression of what instruments say about their owner is NOT going to change.
Treat your tools with respect, people.