I work in the ICT dept for a large organisation. Our mailbox limit is 100MB. Although we do have server side archiving running so old e-mails are stored on less expensive storage outside the exchange mailbox,
Off the shelf SATA hard disks may be cheap. Enterprise SAN storage, not so much. You allow all your users 1GB instead of 100MB, organisation with 1000 users, that's an extra 900GB of storage that you need. Plus another 900GB at your DR site, plus an extra 900GB of backup space times however many full backups you keep. Costs quickly add up.
Due to legislation governing how we record data, we have group policy that prevents Outlook from using pst files. This is to allow central searching of ALL data. We also found that pst file problems caused a significant quantity of our service desk calls.
Our reasoning is that most people keep stuff out of habit rather than out of need. By limiting the space available they are more likely to keep only what they need. Personally, I hit my mailbox limit last week. It took me half an hour to sift though it and I finished up deleting all except 3 e-mails. This is the first time in over 2 years that I have had to do any tidy up of my mailbox. (I also discovered the reason I had run out of space was becuase Outlook had subscribed to a few RSS feeds when I installed it and I hadn't noticed)