Chip prize draw account

Sillychuckie

The November prize pot is 1 x £10k grand prize, plus 250 x £10.
SO that is a total of £10k + £2,500k...    £12,500 (in 1 month), which is equivalen to 150k if you scaled it up by 12 months. 
So if the total amount of money held with Chip Prize Savings, across all customers, was say, £10 million, then that works out as a 1.5% effective return rate.

The trouble I have with the account, is that:
1. The prize pots differ each month, so my figures above are nonsense.
2. They do not publish how much they hold in total, so its impossible to calculate whether its remotely good value or not.

With PBs, and heck, even with the Lottery, you know what the odds are. This seems completely opaque.
In October, they paid out 8000 £5 prizes, just so they could market your odds as being "3.5 x more likely to win than Premium Bonds"... But then in November, the payout # fell from 10,501 to 251.

December looks better again, but without knowing how many total entrants there are and therefore being able to calculate your odds, it is still basically a guessing game.
I can't see any incentive for them to be generous?

So who would buy into this, and why?

I'm very confused by the concept that you have to Withdraw ALL your funds, in order to claim the 'Bonuses'.
So I'd ahve to take out 50k to cash in my £10 win...  but then could buy £50k again the next day (although I'd be out of the 'ticket accumulation period' for 3-4 days in the interim. Can someone help me understand why they might do that?