It's not their personal money they're spending though, it's the club's. From their point of view, Man Utd is a fattened cash cow to be milked, hence they've been able to make shit loads from dividends.
The football side of the business is money poorly spent, but it hasn't hurt the underlying value of the asset. If anything, all they've done is engineer a situation where they can take millions each year in dividends, pay off zero debt, top up the interest with the rest of the revenue and simply coast along ad infinitum. All the while the underlying value of the club has absolutely sky rocketed since their takeover, because the whole club is geared towards maximising the brand globally and decoupling the football and business (something that was their main aim very early on).
Honestly, I reckon billionaires everywhere are jealous of the Glazers in their purchase of United and their ability to rinse it. The original cnut never stepped foot in the actual stadium apart from maybe the day he bought the club, which shows how few fecks he actually gave.
Long term it's pretty unsustainable, although I actually thought they'd have sold by now about 10 years ago. Whenever they do eventually decide to feck off, they'll make a hefty profit on their original outlay (£790m). Decent business, all things considered. The fans have just been fecked in the process.