Aside from the security issues involved with foam instead in OSB (And these are valid especially in the case of vinyl siding), there are insulation issues in favor of foam. When we think of an R-19 wall, we usually think of 2X6 w/ R-19 batting put in it. Aside from the point that a 6 1/4" batt in a 5 1/2" space is not R-19, the value stated is the resistance through the center of the batting. At the 2X6 the value is only as good as the R-Value of the wood (Roughly about 1 per inch). When you install foam over the entire wall, your increasing the R-value, not only at center stud, but also where it needs it most, over the studs & plates. At those points on a 2X4 wall you would be doubling the R-value. When this is considered, a 2X4 wall w/ R-13 batting and 1" styrene board with a R of 5, producing a total of R-18 @ center stud would outperforming a 2X6 R-19 wall (Actually with a R of between 17 & 18).
By the way, they do make a R-15 3.5" fiberglass batt. It's very expensive!