It just means that there will be no more scripted events that occur after 1204 (turn 208). Just like in vanilla M2TW or in SS, you can choose to play on after the end date. However, you will get no more new scripted events and (I think) you won't get a winning message if you complete your campaign after this limit. The fact is, that relatively few people make campaigns that last for 200+ turns; many will have either finished or given up before then. Have you ever seen late musketeers? I haven't =)
The biggest advantage of limiting the time scale of the mod is that a lot of space in the EDU can be freed up for the creation of new units and as a consequence, new unique factions. The rosters of each faction have been expanded to include a larger variety of units so you won't see endless spear militia vs spear militia battles.