In our original design we also preferred to have both available. But at least for now we are moving all instanced Raids to be tuned for 20-players. We may add the 10-man versions back in post-launch but I'm not making any promises. This is has not been a quick decision. We have been discussing this option for awhile. Below is the "short version" of some of the main reasons we changed our design.
1. We're providing significantly more raid content then we originally scoped. This has allowed us to provide parties of both sizes content that is uniquely tailored to them.
2. We took a close look at the feedback and questions we received from our player-base AND took a very close look at "other-game(s)" that provided content with multiple raid sizes. Our general consensus is that there can be significant negative effects on the players and guilds as they try to both maximize their gear acquisition and straddle the two different raid-sizes.
3. Making this change streamlines our design process. There is a significant time investment in trying to balance difficulty and rewards to the two raid sizes. What we didn't want was there to be an "easier path" to progression. We knew this ahead of time and were prepared for it. But now this frees up our resources to provide a higher-quality experience and more content overall... or as I like call it "MoreBetter".
For those of you in smaller guilds we totally understand your concerns. We have several designers on the instance team that play in 10-man guilds. But we do hope that you can participate in the 20-man content either by joining forces with another small guild or perhaps growing the size of your own. Even if you don't there will still be a lot of 10-man content to enjoy throughout Rift.