I think it's fair to have a set requirement for arguments and if debate.Org wishes to have such policy then that's fine. 50 words is rather short and if you can reach your point within that, excellent, if you wish to exceed the 50 words, you can if you want.
If your reasoning for supporting or being in opposition to something cannot fill 50 words, then it is clearly lacking in subsistence. While an argument can be summed up in few words, it can also be expanded and explained, so that it isn't just a bunch of buzz words, catch phrases, or bumper sticker quotes that don't serve to persuade those with an open mind. While we can argue if it should 50 words, or 40 words, or 60 words, the general principle of the argument is the same.
The minimum of 50 words stops many people from posting arguments with little support or no explanation. Without the 50 words, many people would just respond with 'Yes' or 'No' and there would really be no argument in the first place. There would also be a lot more people trolling.
There should be options to only vote in order to get more accurate opinions. People don't want to sit around typing long responses so instead they don't vote and their opinion isn't recorded. There should be an option to comment with a 50 word minimum, and an option to simply vote without a comment.
Some arguments are simple and don't need 50 words or more to be spelled out. This minimum word count can lose a lot of people who otherwise would participate at this site. In business school I was taught KISS. To Keep It Simple Stupid! Don't write more than is absolutely necessary since time is money.