[eosrei]

The development process is much less complicated when you only consider a single OS. The answer to: "How do I do this?" is always the same; a significant time/brain savings.

[Argorak]

Also, if you develop for multiple clients, projects don't bleed into each other.

"Well, client X still uses PG 9.1, but I accidentally used a 9.3 feature, because thats what client Y uses."