I would imagine you're talking about hypermile techniques (or whatever it is called) and the reason they try to avoid coasting in EV mode is simple : they feel whatever energy you use in EV mode will have to be replaced later by using ICE and efficiency of energy conversion from electric (actually chemical, but let's make it simple) to mechanical (driving in EV) and later from mechanical to electrical (recharging batteries afterward) is not 100%, but it is about 90%-95% and therefore, since you have 2way conversion netting 80% to 90% efficiency, some people feel they get better MPG by keeping the ICE on all the time and never use EV mode. I don't use this type of driving, so I don't have personal experience, but my gut feeling is they're partially wrong and in some cases it's better to coast in EV and in some it's not depending on particular situation (for example driving up the hill in EV will deplete battery too quickly, driving down the hill with ICE running wastes energy).
I don't think Haroldo wild guess is even on the same page, so to speak, no offense intended, please.