I try to 'think' in terms of 'capital' holdings not margin..
Leaverage does, of course, add risk, and prospective return, but it's a different choice whether to do it to the one on whether to buy and sell shares or use cfds for short term trades.
Given Ace's stated risk profile, a pot of £100K could be used to support trades on long CFD's of up to £10K for up to 2 months more economically than trading actual shares, ignoring the spread issue.
I'd consider a portfolio of exclusively long positons as greater risk than a mix of longs and shorts with net nil exposure, though the army of buy/hold investors don't agree.
Few consider a proper portfolio balance, and many think it safer to cling to a single share they have had success with than diversify, despite the factual risk evidence to the contrary. FTSE 100 Co's can't go bust can they?
Energis,British & Commonwealth, Polly Peck, Marconi...