The main thing I found is that you will make more if you don't lay the bets off in the long term.
Yup, you're taking value from the bookies by virtue of there being a free bet*. Both sides should usually be -EV because of vig, commissions etc.. but the presence of the free bet makes things +EV at the bookies, matching at the exchanges is basically just hedging/reducing variance and there is a cost to doing that.
That is right, as (i) if you don't lay off you don't have to pay the commission at the exchange, and (ii) you're generally betting into shortening odds, so over time you tend to win at bookie and lose at exchange.
The second part doesn't matter, all that matters is that the activity at the exchange is generally -EV because exchanges tend to be efficiently priced and so assuming the mid-price is fair, you crossing the spread + paying commission makes things -EV for any given bet/trade there on average.**
There was a guy in the thread earlier (amigafan IIRC) who used to BS that he had a way to structure his bets to make sure he won at the exchanges, if anyone had a way to get a net positive balance at the exchange then they'd not need matched betting in the first place and could ignore having to wait for free bets/promos and simply carry on doing whatever they're doing at the exchanges.
(*Sometimes there are also arbitrage situations and inevitably these involve some bookies being generously priced or mispriced relative to the rest of the market so make the bet at the bookies +EV regardless of the free bet. It's not necessarily a good idea to take advantage of these if you want to keep your account for long at most bookies)
(**Of course if you're an expert handicapper then there might well be times where the exchange isn't fairly priced and you can profit, which in turn helps bring prices back into line)