Any ship will be able to be obtained in-game (ie. bought with in-game currency, aka "UEE credits"). A rare one (ie. Vanduul Scythe) may be hard to find/obtain, but not impossible.
People purchasing ships with real currency are doing it to:
1. Obtain the ships sooner, without having to spend lots of time earning enough UEE credits to buy them in-game. Some folks want to get a jump start on playing the career/story of their choice. Some folks like to collect ships.
2. Support further development of Star Citizen.
Those purchases of additional ships is not required to play. You could purchase the $30 Aurora MR Starter package, and never spend another dime of real currency, if that is your desire. (They even had an anniversary sale for $20 to get in for awhile there.)
https://robertsspaceindustries.com/pledge/Packages/Aurora-MR Star Citizen is crowd funded. There is no 3rd party publisher shoveling money (with strings attached) to CIG. The ship sales for real currency help fund the continuing development, and enable additions/improvements to be made.
Total funding is up around $75 million at this point. There are hundreds of people spread out over multiple studios working on various aspects of this. CIG puts out plenty of information on a regular basis to keep the backers informed of how development is coming along:
https://robertsspaceindustries.com/comm-linkI doubt that anyone needs to feel sorry for Chris Roberts. He is getting to build the game he has been dreaming of for years, without having a publisher handcuff his design decisions or jack him around on meeting deadlines.