Bad Idea
Three have driven down prices, and if you're don't mind a handset being forced to basically use their network when available (resulting in worse battery life) then people still choose them. Entire families can go for some packages, and total monthly cost is not far off half the cost of the 2g+3g networks.
The reason I haven't bothered with them is the bad reputation they gained when they were first available, and the fact the three broadband dongle I was given is no longer allowed to roam on 2G, only use three's 3g network, so on a UK North to South train it is next to useless and only works sporadically for 10 seconds at a time; the software does not handle this and generally siezes up.
However I'm sure it will get better as we're forced through marketing onto 3.5g and 4g onwards, so allowing three to be swalloed up by any competitor would be a bad idea in my opinion. Didn't some or all of the original owners of three sell part or all of the company off anyway?