Secondly, if a dev wants to reduce his exposure to crypto he can immediately sell his MSC for fiat, in the very moment he got paid. What's the problem?
The problem is that you're assuming MSC is more liquid than it is. If you want to sell your MSC, you need to find a buyer, which isn't always easy, and even if you do find a buyer, he may not want as many Mastercoins as you are selling, in which case you may have to lower your ask price.
Reversing your remark would be better: If a dev wants to increase his exposure to mastercoin he can sell his fiat for MSC. This is surely much easier than selling MSC for fiat.
I don't know how many times it needs to be proven that we're going "all in". If the goal is to attract good developers, irrespective of whether *they* are "all in" or not, they should be paid in fiat, or at least BTC.
I think its better to pay developers in MSC and to have the foundation hold MSC. It's a lot easier to trust a foundation or company which uses it's own product than the foundation or company which uses some other product internally.
Ideally you want developers who really believe in what Mastercoin can become rather than speculators who just want a quick buck. I think it should be MSC or fiat for developers but not BTC, not LTC, or PTS. Imagine how awkward it is if the Bitcoin foundation paid people in Litecoins.