Skjalg's argument of laziness may sound ridiculous, but it rings true. Companies should do their utmost to remove all of the barriers between the customer and their product. It doesn't matter if it's a small or a large barrier. You can't impulse purchase something very effectively if you have to think too much about it.
I've been on steam since the beta and I've purchased a grand total of one game using it -- Half Life 2. Why? Two reasons.
1. For new releases, steam is very expensive. £40 for MW2? I've seen it for as low as £26 (Sainsbury's stunt) and you can find it easily enough for ~£33 in other places. I'm guessing Valve takes their cut and the game's regular publishers also want their standard cut so it pushes the price up, but I'm not willing to pay an extra £7 for 'convenience', especially when I see the US pricing. I can buy 4 triple A games on steam, or I can buy 5 online for the same money. No dilemma here.
2. Steam doesn't accept debit cards and I neither have nor want a credit card. If I want to purchase on steam, I need to deposit money into paypal and then pay using that. It's too much hassle for me, plus I don't trust paypal -- too many horror stories. As a result, Valve never gets my money, nor do any of the companies that sell their games via steam. It may sound weird to some people, but I am far happier to go to play.com, order and wait for the best part of a week. It takes longer, but I don't have to do anything other than click two buttons on a website. Ever since I started working full time, I am much more mindful about how much time I waste doing stupid bullshit. Stupid bullshit like logging into my bank, moving money into a pay pal account, then remembering all of my details and purchasing. 3 steps instead of 1. 15 minutes of fail instead of 1 minute of simplicity. I don't want to fuck about on 3 websites to order 1 game.
Now, I'll admit that I'm rarely going to purchase new releases of 'big' games on steam because of the price, but there's been loads of games that I would've purchased in an impulse buying mood (especially around the £5 to £15 mark). However, it's too much effort for me. I cannot be arsed to go find my wallet and move money around etc. Instead, I'll fire up an old game or do something else. I would probably spend an extra £50 a year on steam on smaller titles if they removed the barrier. In these cases, the wait does matter because I want to buy and play something now, so I don't want to order via play.
I don't pirate because
A: I cannot be arsed
B: I'm not a student any more. I don't really have an excuse.