Interesting observation. But MB Trading's "pay for limits" liquidity situation is near ZERO, since the only eligible counterparties they have are other MB Trading retail clients. It was more marketing hype than a real benefit, and I was inside the spread constantly, but the liquidity on their "EXN" was not there.
At Dukascopy I believe it's quite different from MB Trading's pseudo-ECN.
I left MB Trading because of their terrible liquidity situation in Pay for Limits.
However, in order to "add liquidity", I'd say you would have to use PLACE_BID and PLACE_OFFER order types. If you merely use conditional limits, then these convert to Retail on trigger, and are not maintained as live Bids/Offers to add liquidity.
My system, HyperScalper, always uses PLACE_BID type orders, so you are "live" on the ECN, and these do "add liquidity" to the ECN, however small the increment may be.
It would be great if these "live ECN" order types had SOME commission rebate associated with them.
But, alas, they don't

I definitely get price improvement due to usage of these "live ECN" order types, so it's a benefit, but some commission advantage would also be much appreciated.
HyperScalper