A double reed player does not need to be fully dedicated to whatever they choose to play for marching season. I've seen too many kids quit their freshman year (or even between band camp and school starting) because whatever expectation has been placed upon them for marching band seems so overwhelming to them. If they are going to be in the front ensemble, those parts need to be written at an achievable level for them (i.e. don't give four mallet parts to freshman double reed players). Whether they play percussion or a secondary wind instrument, they need to be encouraged to play their primary and the director should ensure that time is given for them to do that.
If we fully expect them to mature as double reed musicians, it is our responsibility to make sure they have the time to continue those skills. Expecting that they will do that on their own is not fair to their personal time. Yes, they should practice outside of class, but most marching bands have lots of time commitments. Expecting a 14-year-old to manage their time without any assistance is asinine.