The two reasons I see for not wanting Solavei are 1) people who don't want to be part of an
MLM (as well as being a new company) and 2) the high start up costs. From what I recall, the normal price to get started on Solavei (if you bring your own phone) is $130. I understand that have frequently had special deals that take $50 off that price, but even with that you still have an $80 payment for your first month.
Now, I'll agree that T-Mobile's $50 and $60 prepaid plans are overpriced and I'd really like to see T-Mobile make them more competitive. OTOH, there are other MVNO's, Straight Talk being one example, where you can still be on T-Mobile's network and pay $45/month, with only $15 extra to buy a SIM (which you can frequently get for $5).
As for the Value Plan, it is a great deal if you don't mind having a contract. As mentioned earlier, you get the advantages of a postpaid plan (data roaming, etc.) while paying similar prices to prepaid. If you have a family and multiple phones, it is actually cheaper than prepaid. Then, on top of this, you can get the "cheap" phones, in the form of a no-interest loan.