Surely this comes down to how well you can match your bets.
Well potentially - if you're able to otherwise make money from betfair via +ev bets or get lucky, otherwise not really.
Of course you might find a straight up arb too (though in that case it would more likely be a dodgy price at the bookies)
I think you had a flawed example previously (you had a dodgy betfair price then based an argument around it) and have been stuck on that. I presume you've thought about the questions posed previously?
If not then consider a coin flip - this should illustrate it
lets say paddy power offers the following:
Heads 1.8
Tails 1.8
the EV is (8*.5) + (-10 * .5) = -£1
(the vig is 11.1/111.1 = 10%)
The true odds for each are 2
Now suppose betfair has a more efficient market around the true odds 1.95 : 2.05
well we could lay at 2.05 for a QL of £1.22
note this is a bigger loss than the EV of the paddy power bet
note if betfair allowed us to lay at 2 (i.e. at the true odds, a 0ev bet) then we have a QL of £1 - identical to the EV of the bet at the bookie... if we were able to lay at less than 2 at betfair... on a coin flip... well we don't need matched betting, we've got free money from betfair already... that would be a +ev bet!
Note the EV of the bet is unchanged - and generally betfair doesn't give away free money easily, you're generally going to have some additional loss thanks to betfair.
Hopefully it is abundantly obvious that a) if you carry on betting on coin flips at paddy power at 1.8 then you'll lose money in the long run
b) if you carry on laying coin flips at betfair at 2.05 then you'll lose in the long run
You make money from the free bet, that money comes to you via the bookie, the effect of laying at betfair is to reduce variance, it costs you money, it is like insurance. If you were able to not lose money at the exchange then you might as well just make money from betfair!