Yes, I think you're making a good point. It's a complicated issue, because there are downsides as well as upsides.
If you tax a husband and wife together and one of the partners has been out of the labour force and is thinking about perhaps taking a job, their effective marginal tax rate is the marginal tax rate of the family. If the primary earner is earning $70,000 or $80,000, that is a high marginal tax rate; whereas in our system, when the secondary earner thinks of going into the labour force, the first $10,000 approximately is at a zero tax rate. Actually, that's not quite right, but they enter independently, so they enter at the bottom of the tax schedule and there is more incentive for them to enter the labour force, as they're taxed more generously.
Another point to make is that as tax structures are flattened, this becomes a less serious issue. Now, if you actually move to a flat tax, the issue would largely go away. So I would hope that we would continue to flatten the tax structure in the future, and maybe then it won't be so important.
These are good points that you're raising, ones that are well worth examining, I think.