I don't think that's the case, I think that a big bonus such as this would generally tend to amplify whatever one's pre-existing sentiment toward their work place was. If they grudgingly go to work and do so only for financial reasons then a big bonus could lower morale. It could lead to a temporary emotional high which comes crashing back down once it's passed into a new low as folks re-evaluate their enjoyment of their work, calibrated to a new scale. More so, being more financially secure could lead to more pent up hostility coming to the fore, as people stop being polite or stop cooperating at work (the "fuck you, I'm fully vested" mentality).

However, if the work place is a good one and the people there honestly love working there then I don't think big, irregular bonuses are at all a problem. They will still go back to work, and they will still enjoy it, and perhaps they will be a little bit more comfortable in the feeling that they can choose their work based on how much they enjoy it instead of out of a perceived financial need to try to climb the management chain or such-like.

Also, the problem of trying to figure out whether candidates are just there for the money applies even when the company is merely offering industry standard wages.