What we usually say to people is that if you only have 10, 15 minutes, then do it, that a few long sessions once or twice a week is probably not as good as 15 minutes every day. The regularity of the practice gets us more in tune to doing it, and as we get more in tune to having it as part of our day then the sessions can increase in length. If we set out to do long sessions, people tend to get discouraged and say, "Oh I haven't got time therefore I won't do it!" As well, the longer sessions may be long but they could become less and less frequent, and you don't get the regular feeling of being there trying and trying. Five or ten minutes of reflecting about the five reflections is something that keeps them in your mind much more regularly and you're less likely to forget.
We had one old student who just couldn't seem to get the daily sessions going over many years. He had an interview with Steve one day, and Steve encouraged him to do at least five minutes each day. So the old student decided he would meditate five minutes a day, and it extended into ten minutes a day. After a while he had gone traveling with someone and he missed his regular sessions. Then he was really inspired to do it again because he really felt a difference in his consciousness when he was doing those five or ten minutes. It became part of him and he started to increase the time.
So I don't think you're tending to devalue the practice, you're just trying to remember that it's important to do it regularly just like with physical exercise. Sometimes if a person starts out too ambitious and does an hour of physical activity and gets sore muscles, then they may say, "Oh, it's too much!" But if they start out at a more obtainable level, fifteen minutes each day, and get used to that level, then they can start to increase that level, rather than get the idea that it's too much. The more we increase our capacity to do these short periods, the more it becomes part of our life, and something we wouldn't even consider missing like cleaning our teeth.