Yes, the minimum is 50 BTC. It could be lowered some (say maybe to 25 BTC) if we get enough requests for that. The idea behind it is to be able to place large orders without others knowing, since if other traders know about your large buy/sell interest, this might cause the market to move against you and make it harder to fill your order. So it's intended specifically as a service for placing large orders.
Yes this a good reason. But as far as I know, other exchanges have no special minimum limit for dark orders.
Because you can add the argument: Also for small orders it could get difficult to get filled, when there are alot of bots active.
Some aggressive bots are written that way, that they will always overbid you 0.01€ or simular. By making your order a dark order, these bots would not know about your order and would not overbid you.
So I think there should not be a limit, because both arguments have the same "value"
I believe LakeBTC offers a dark pool, and their minimum order is also 50 BTC. Remember that the dark pool is a separate market with a separate hidden order book. It's not intended as a market for bots to run on. The sole purpose of the market is as place where people can place large orders discretely. If you allow small order size, then people can now probe the hidden book with small orders to try and figure out where the orders are, so it undermines the privacy somewhat. One use case for the dark pool might be where someone puts up 10,000 BTC to sell and has previously arranged with someone else to put up a matching sell order to take it. Both parties want this to happen in private and are willing to pay an extra 0.1% fee for the privacy.
But there is a different product that is also useful, even more useful to some, and offered by some other exchanges - namely hidden orders that go to the lit pool (i.e. the normal visible order book). This type of order would be useful for dealing with the bots on the lit pool and here I agree that it does make sense to allow for smaller orders. This is something we are also looking at offering, but we consider it to be a separate product. So, stay tuned, and maybe you will be able to place hidden orders to the lit pool if that feature appeals to you.