A valuation allowance is never a good thing.
Losing your deferred tax assets means that the auditors, based on principles such as GAAP, believe that your company will not be profitable fast enough to make use of the deferred tax assets.
Why is this a bad thing? I would have to look at the financials, but this is a bad thing because losing your deferred tax assets could lead to going concern questions. A going concern question calls into question the ability of the company to continue in existence for even the next year.
However, since Sony had an operating profit, this shouldn't be all that ominous. It also sucks that the loss from writing down DTAs all comes in the same period, when it was really more attributable to the last three years.
Keep in mind that just because a company has an operating profit doesn't mean they are healthy. If you have bad Net Incomes but good operating profits you could be looking at a change in ownership at some point. Most of all, and the reason I wrote this post, is that losing deferred tax assets is not in any way shape or form a good thing. Investors would sell the stock based off of that fact alone.