In fairness, using an arb opportunity to lay one that you really want to lay can be very handy - it takes all the effort out of it! If a firm is selective and lays strictly to the level they want to reach in the book then cut the price it can be very useful indeed, so long as your traders/compilers are worth their salt!
As Gearoid says, no point in looking to trade unless you can do it full time. There's no point arbing unless you plan to sit up 24/7 doing so, as with each arbing opportunity you'll be lucky to earn enough to buy a Kit-Kat - and in arbing with fixed price firms you'll very rapidly have all your accounts boshed and struggle to get even pennies on, anywhere. Quickest way to have your accounts closed.
In-running can also only really be done seriously by those who do it full time, are dedicated and know exactly what they are doing. It is a big money business and those who do it professionally are constantly looking for an edge; even the in-running shops aren't worth anything any more as even their satellite links lag a second or two behind the oncourse pictures, those precious couple of seconds being vital. Those who are taking it seriously have to do in oncourse now, where they can.