Hypothetical situtation. I am trying a horse for sale. Seller rides horse. I decide I do not want horse. Do I get on and ride anyway?
Yes
Anything is Possible: I could change my mind. Highly unlikely, particularly if the horse has physical issues, but theoretically possible.
Learning: It would be good practice for me in evaluating strange horses.
Counterphobia: One should do the things that make one nervous.
No
Bad Manners: Definitely not if the horse is thundering around the ring doing his impression of the Oncoming Storm.
Exertion: More work for the horse, especially in heat, cold, rain.
Liability: What if something happens while I ride? What if nothing happens and the horse comes up lame for another reason entirely & I get blamed? Granted this is an accepted risk of showing &/or trying a horse, but why expose myself/the horse if it is not necessary?
False Pretenses: Good practice for me is just using someone else’s horse for my own ends. (Update: and wasting the seller’s time.)
Therefore
Ride or not?
Virtual Brush Box 
Or not…. a polite “He’s lovely but he’s just not my kind of horse” worked for me.
I say go with your gut. Unless of course your gut needs a little shove. I tend to hesitate. For one, I’m not accustomed to riding in front of an audience. I know that sounds dumb, but to me there’s nothing worse than having to ride a horse you don’t know or trust in front of people you don’t know or trust. So if your gut is concerned about physical safety I’d say pass, but if your gut is just being a big wuss then I’d say try to suck it up and give it a shot. I’ve test ridden horses that I refused to do anything more than walk or trot. No shame in that, but it kind of told me that if I didn’t trust the horse in it’s own environment then it was probably going to be too much of a “project” for me at home. Sometimes bad weather or footing can be your friend. I’ve passed on test riding more than one horse by saying I didn’t think the footing or conditions were safe that day. On the flip side I do think it’s good to test ride a variety of horses. I think it helps you narrow the field and gain more confidence in the whole selective process.
When we were selling our farm (and we ultimately sold it ourselves, without an agent), we had about a dozen people come to see the property. About half of them drove down the driveway, parked, took one look around and left (without even going into the house). The buyers knew they weren’t interested right off the bat, and wasted neither their nor our time. I think it’s okay to make a decision on the spot, and move on.
Good point, Added.
Three comments on Yes:
Comment 1
A Stable Master who was about to be executed said to the King “If you give me a year, I will teach your Horse to talk.”
The King was delighted for he loved his Horse very much. “I will give you a year.”
That night, the Stable Master’s wife said “Are you crazy? Horses don’t talk.”
The Stable Master replied “I know but in a year, the King might die, I might die or the Horse could learn to talk.”
However, in his case, the pros outweigh the cons. In your case, just the opposite.
Comment 2
If you do find a horse you trust and want to buy, your feeling should be strong enough and clear enough that the relationship between you and the horse will be your focus, not the bystanders.
Comment 3
Referring to your recent off-topic blog, listen to your gut. Is she is shouting “this makes me nervous”, listen to her. Fears and caution keep us alive.
Conclusion: no.
Ride.