The point is DT, spread-betters who have never traded DMA have no reference to base their assumptions on. I trade both a DMA account and a spread-betting account so I can at least give a fully informed opinion. The evidence given so far, like 1) Slow fills and 2) slippage...can and do happen on DMA depending on when you enter. I've seen things on DMA that would make your eyes water had you entered at that time, and I'm certain if a spread better had they'd be bitchin' about their broker.
The point is, spread-betting 'scalpers' attempt to catch price 'spikes' and then moan when their order isn't filled...or they get a bad fill and then have insult rubbed into injury with slippage. All I can suggest is you get a DMA account and see the real world before trying to use your spread betting broker as the scapegoat for your losses. Then, spend some time learning how to scalp properly because I've got news for you, trying to catch a few points on every price spike is the surest road to poverty...whether you are trading DMA or spread-betting.