We see a bit of Rails in F500 shops but it's mostly SAAS companies.

Interesting.

SaaS companies are typically either smaller (hence don't have large budgets) or prefer full-time software engineers.

Would you mind, if you have time, to extrapolate why these companies prefer consultant/contractor or even a shop?

If you can budget an FTE, you can budget a contractor. Lots of SAAS companies have hundreds of employees. I don't know how else to respond to this.