Is it acceptable to schedule a meeting without a clear agenda?

Is it acceptable to schedule a meeting without a clear agenda?
Agendas and meetings go together like Ant and Dec, like Batman and Robin, like the sun and an ice-cold pint of beer. You can’t have one without the other. Okay maybe don’t hold us to the beer comment, we by no means are telling you to drink every time it's sunny! However, agendas are very important to the smooth running of a meeting, so we asked 200 people their thoughts on whether they find it acceptable to schedule a meeting without having a clear agenda that allows participants to prep having known the topic prior to the meeting.
Through our results we discovered the following:
Yes: 17%
No: 65%
It depends: 19%
However, some of the comments we received seemed a bit more varied on response.
A common theme seemed to be that it largely depended on the formality of the meeting. Best summed up in our comments area by one participant with the following:
“Depends on the nature of the meeting and if sensitive issues are being discussed.”
I think this is a very valid point and holds a lot of weight in this discussion as an agenda for a team catch-up seems overkill, not to say that they shouldn’t be made aware of the nature of the meeting in advance though. Participants of the poll mentioned that relationships with other meeting participants also play a big part in this discussion. They state that “If I am invited to a meeting where I know the organiser well then, I also know it would be important for me to attend, and that it's likely they have an informal agenda or points to discuss.
"If I don't know them well, then I would feel a bit confused and unprepared as to why I was invited without any knowledge of what the meeting was about.”
It seems that in general, participants agree that in a formally set meeting, it is important to have an agenda set out prior to the meeting to allow sufficient preparation from participants. One participant summed it up brilliantly by stating “One of the most important aspects of meeting etiquette is to respect and cherish everyone's time, so having a clear agenda is really important. It doesn't have to be very detailed so there is still flexibility to explore new ideas, but there should be a sense of what the meeting is about and what all parties hope to achieve. Basically, you want people to leave feeling 'that was useful'. Unfortunately, all too often people leave meetings feeling like it was a waste of their precious time.” As you know, keeping meeting etiquette is important for having a smooth run in any meeting and many people see having a clear agenda as a part of good meeting etiquette. Just remember, your agenda doesn’t have to be the most in-depth detailed agenda in the world, it just needs to set a basic outline of the aims of your meeting so you aren’t blindsiding anyone with a meeting topic that they haven’t been able to prepare for.