IMHO, I think that the ranking system is over emphasised. I think what is important is our journey for improvement in our aikido and what we do to improve our practice of aikido.
That is not to say that ranks do not have a place. They do but not as a way of comparison with other aikidoka but as a means of measuring your own progress against a standard. Comparisons are not going to be possible unless the same syllabus and grading requirements are used. I'm not sure that comparing myself to other people is that useful. That said I have often said to myself " I would like to know how this person does what he/she does because they do that really well".
On a more personal note I regularly train at two dojos. Initially I joined the second dojo on a casual basis to get some extra mat time. After two years, I joined as a full member and the shidoin and sensei at the second dojo accepted my ranking given to me at my first dojo, though I had no expectations that I would have the same rank.
As to the safety question as an argument for the ranking system, I think its a valid concern. IMHO, as responsible aikidoka, we need to take personal responsibility to train safely and take care of Uke regardless of their experience. When I visit another dojo to learn a different way of doing aikido I contact the shidoin or sensei to ask if I can train with them and to provide a little of my aikido experience and the lineage of the Senseis that I train under and ask them a few questions regarding etiquette for visitors. I dont tell them my rank unless they ask me because the grading standards I went through is likely to be different to their grading standards.
In addition, their way of aikido is likely to be different so I'm going to be a "beginner" all over again. Its the diferences that I am interested in learning when I visit another dojo.
Happy training all