Both Poloniex and Bittrex have withdrawal limits, long KYC process and payment verification, it can take weeks. Changelly almost has no amount restrictions for exchange. For the big crypto-crypto trades we usually provide better rates, even if you exchange manually on Poloniex. As we have a pretty small trading fees on our verified corporate accounts on Polo and Bittrex, our exchange partners. We split the deal to a few smaller orders and trade right near the spread to give you the best available rate on the market.
As for the withdrawal fees, recently we've implemented merging bitcoins transaction to reduce the fees and provide super fast blockchain confirmation. The same as Poloniex does.
I have personally used both Bittrex and Poloniex - yes they have some restrictions but for most of the traders it is within their own limit. Anyway it is not advised to keep to many coins on an exchange, but rather trade slowly in short parts. Since you buy and sell coins on Bittrex and Poloniex which means that your rates are higher as you need to earn some money on this, same with shapeshift, their rates are higher.
It is way cheaper to buy on bittrex, polo , cryptopia or any other exchange than to use your service, one time I looked at your prices to buy some coins from fiat and exchange RATE is just to huge for me to afford your service, so I went to another exchange website. For some they might find your service convenient, but thats not me, sorry.
You may merge all in one transaction but still the amount of the fee that this guy paid is a lot, yes it was sorted out. But just imagine that all of the people to whom this fee was applied came to ask for refund? It would prob be to much to pay.