In modern times, marching band for people too stupid to spell properly - or care enough to use a spellchecker. Such people are usually woefully ignorant about a lot of other things too. Sometimes this is due to the presence and influence of mediocre teachers, purported leaders and even predators who use children for reasons of sexual or financial exploitation. All of these are very bad signs of something no one decent should want to be involved with. Amusingly for "corp" critics, "corp" is also the abbreviation (without the period) for corporation - and corporate exploitation is often one of the reasons for the ignorance and lack of care such mispellings reflect.
The use of the terms "drumcorps", "drumcorp", "drum corp" and "corp" are all signs of ignorance and a lack of respect for drum corps, drum and bugle corps and other genuine marching musical history.