Some 'possible' reasons to use different client and server frameworks:
1> Decoupling. Add APIs / more clients in future
2> Effective use of skills. Many developers specialize in one of the two.
3> Parallel development of client and server
4> Future client side caching, tracking etc. (can be solved with a lot of server code)
This is not to say one should not do it, use cases differ.
1> You can find many analysts and many more engineers with Hadoop skills.
2> Hardware & power costs - are not huge (assuming hadoop is slower) - at least till you reach a massive scale.
Charging for something when you know you are going to give away = Not a good idea.
If there is existing market that you can reach, you can go with paid option. Else start with open source and then add either paid version (more features) or supported version (dual license). Service in cloud is another option.
1> Decoupling. Add APIs / more clients in future 2> Effective use of skills. Many developers specialize in one of the two. 3> Parallel development of client and server 4> Future client side caching, tracking etc. (can be solved with a lot of server code)
This is not to say one should not do it, use cases differ.