All mechadendrites are listed as being good quality. Does that mean the weapons on them are also good quality/gain the benefits of good quality weapons? This doesn't really matter for the ballistic mechadendrite as is since it is already reliable but the melee weapon parts of the others would get a nice little bump to Weapon skill.
I had never really thought about it before until Enemies Within came out and the Selvanus Binary home world ability got me looking at what crazy things I could do with increasing item quality and I realized I wasn't sure how it would affect the mechadendrites. If they do count as good quality weapons standard then the selvanus binary ability would allow them to be increased to best which would give a big boost to the melee attachments and would even benefit the ballistic mechadendrite.

I have never had the quality of the mechadendrite have any effect on game play. When thinking of something like a utility mechadendrite, a good multi-tool does not do anything special. It can count as a good quality cybernetic, but the attachment, such as a hellpistol, would not receive such benefits. I would also call into question the source of the mechadendrite. If it was granted as an act of heroism, maybe it was shiny with a good-quality weapon on it, but i would say that most of the time the weapon is common quality.