The foam would flow into areas you may not get to blow in cellulose. It might also strengthen the structure, possibly an issue if you have earthquakes or are subject to hurricanes. It would likely provide less air intrusion, which has both good and bad issues. It has a higher R-factor. It would be a major pain if you even needed to run new electric or plumbing through the wall! So, you probably get what you pay for. Utility costs are only likely to continue to go up.