One method of training that is quite popular amongst certain groups is fencing slowly, possibly using minimal safety gear, or even sharp swords, during sparring. The use of minimal gear or sharp swords will of course force the fencers to spar more slowly for reasons of safety. Alternatively, some fencers may spar at higher speeds but at the same time may avoid striking with any force. This is often because fencers may have an idea that striking quickly, with strength and with force is somehow incorrect.
Personally, I do believe that sparring with minimal gear, and sparring at slow speeds is very important and useful. However, if this becomes the only or dominant form of training, then this may represent a problem. A fencer who never fences at high speed with force is a fencer who does not know how to deal with an opponent who will fence against them at high speed with force.
Others have outlined the problems of never fencing with speed and strength, but I wanted to take a further look at this issue from the perspective of advice given in some of the longsword treatises.