Damn. I feel like a real dick now . . .
I wouldn't. Horde mode is fantastic fun, but 50 waves of it can be a big time commitment over several hours, and getting stuck on a boss wave can quickly get tiring. There's no reason to continue to slog through it if you feel you've hit your limit for the day, and are simply no longer having fun playing it.
I can't really think of a good reason why Coalition didn't want to allow the option of mid-game joining in horde mode. If certain people are against it then simply put a setting toggle in Options to prevent a player from matchmaking into an in-progress Horde mode match. As it stands, there are a lot of public matches where people drop out well before even hitting the halfway point of Horde. It's certainly unfortunate for the folks who stay in the match, but at the same time, I don't think it's fair to force people to keep playing purely out of a sense of 'obligation' rather than 'enjoyment'.
Don't punish yourself for a design decision the developer could have corrected by allowing someone willing to take your place in the match.