I can see both sides. I suppose the risk is that people risk larger amounts than they normally would for the sake of being able to trade. The main issue is for stocks which aren't very liquid. A stock could be 80p when you place the trade and then end up at £1 by the time it eventually goes through. Especially as it's a service that was previously available.
On the other hand £100 doesn't feel like much, and if you're loading in £500 a month as mentioned above i'm not sure why you'd spread yourself so thin to not be able to meet those limits. I wouldn't say my "play account" is huge, but i tend to just have 3-4 stocks on the go at once in it between £100 - £200. As
@Russinating said above. Anything less and it just feels pointless. If i'm up 20% then even at £100 it's only £20 which barely feels worth it as it is. Never mind anything less.
Although i tried a trade on something yesterday and it said the minimum limit trade was $250 which is considerably higher than they suggested.