If I'm a market-maker, I want to trade with retail orders or slowly sliced institutional orders that are trying to rebalance a portfolio throughout the day. When I sell stock to them, the price doesn't move very much in the short-term and I have a chance to buy back on the bid to make a spread. I don't want to trade with aggressive arbitrage or stat-arb traders capturing short-term price inefficiencies, hedge funds that want to buy huge blocks of stock all at once, etc. If I sell stock to them, the price ends up moving up more than the spread very rapidly and I lose money.
When trading on a public market, I have no way to control which of these groups I trade with. I have to employ countermeasures to predict whether it's likely that I'll trade with an informed or uninformed trader and adjust my market accordingly. This is why you see market-makers rapidly flash their quotes for seemingly pointless reasons (e.g. I may adjust my prices in an airline when oil futures move) or flip out of positions aggressively when their model believes they were hit by an informed trader.
Now if I'm able to trade on a market where I know I'll only trade with long-term fundamental traders, I can quote much larger, tighter, and more stable markets. This is good for the institutions since their execution costs are lower than going to the exchanges. It's bad for arbitrage traders and hedge funds with short-term information since they effectively pay higher execution costs in a tiered market (if more "good" order flow goes to dark pools, less reaches displayed markets, and spreads on displayed markets will increase to compensate). When people want to end dark trading or internalization, they really want to have retail and institutional flow cross-subsidize fast speculators.
Ironically, it's probably HFT market-making firms who were the most impacted by Barclays giving other high-alpha HFT firms manual overrides to put them in a lower tier. If anyone would be excited at the promise of removing high-alpha traders from the pool, it would be market-makers who want to avoid being "picked off", not institutions who have a much longer time horizon.
If I'm a market-maker, I want to trade with retail orders or slowly sliced institutional orders that are trying to rebalance a portfolio throughout the day. When I sell stock to them, the price doesn't move very much in the short-term and I have a chance to buy back on the bid to make a spread. I don't want to trade with aggressive arbitrage or stat-arb traders capturing short-term price inefficiencies, hedge funds that want to buy huge blocks of stock all at once, etc. If I sell stock to them, the price ends up moving up more than the spread very rapidly and I lose money.
When trading on a public market, I have no way to control which of these groups I trade with. I have to employ countermeasures to predict whether it's likely that I'll trade with an informed or uninformed trader and adjust my market accordingly. This is why you see market-makers rapidly flash their quotes for seemingly pointless reasons (e.g. I may adjust my prices in an airline when oil futures move) or flip out of positions aggressively when their model believes they were hit by an informed trader.
Now if I'm able to trade on a market where I know I'll only trade with long-term fundamental traders, I can quote much larger, tighter, and more stable markets. This is good for the institutions since their execution costs are lower than going to the exchanges. It's bad for arbitrage traders and hedge funds with short-term information since they effectively pay higher execution costs in a tiered market (if more "good" order flow goes to dark pools, less reaches displayed markets, and spreads on displayed markets will increase to compensate). When people want to end dark trading or internalization, they really want to have retail and institutional flow cross-subsidize fast speculators.
Ironically, it's probably HFT market-making firms who were the most impacted by Barclays giving other high-alpha HFT firms manual overrides to put them in a lower tier. If anyone would be excited at the promise of removing high-alpha traders from the pool, it would be market-makers who want to avoid being "picked off", not institutions who have a much longer time horizon.