a suspension stem only provides an increase in comfort to the rider’s hands/arms and a portion of the upper body - it does not provide the benefits of a suspension fork
a suspension fork suspends the front wheel / bike and improves the ability of the bike / rider to track and travel over rough terrain
Had my friend come up with that rebuttal in our discussion of suspension forks versus suspension stems, I'd have pointed out that that argument makes sense for motorcycles but not for bicycles, where the rider greatly outweighs the bike.
My off-road bike with the Softride stem installed did track and handle better than it had with the original stem. After we swapped bikes on a couple of rides, another friend of mine was surprised that he could ride my bike as fast on single-track as his own suspension fork-equipped bike.
Of course, that was back in the early '90s, when suspension forks had barely an inch or two of travel.
Anyway, here's an extract from
an interview with Thomas Frischknecht, the guy who won the mountain bike World Cup on a bike with a SoftRide suspension stem.
(Frischknecht mentions a fact that I'd forgotten: in addition to the World Cup win, both the men's and women's World Championships had been won on bikes with Softride stems---results that effectively put to rest any claim that suspension forks are inherently superior to Softride stems for off-road performance.)
ST: In between running a rigid fork and eventually using a Rock Shox, the Ritchey team relied on Softride (suspension) stems. Do you think it held you back at all?
TF: You have to understand that a suspension fork in 1993 did not have the same to offer as they do today. Those with less than 60mm of travel did not do the job. At that time, the Softride stem was a good alternative. Henrik Djernis and Ruthie Matthes won World Championships using them, and I won the World Cup overall on Softride. We were faster than the riders with suspension forks.