Obviously training with one or a few selected partners is:
1. Not so good for your technique.
2. Impolite towards the other dojo members.
In our dojo, you have to change the partner with every change of technique. I have seen in some Iwama dojos teachers pairing students together, which I find is a good idea especially in beginners classes.
I have seen some people truly attached to eachother in an inseparable way, and I honestly find it disgusting. It is aikido that we're learning after all, not Tango.