I'm not sure how making a sheet cake can be negative for one's business, but speaking for myself, I can't imagine NOT doing sheet cakes.
Sheet cakes are quick, and relatively easy to do for me. I don't make the "typical" grocery store sheet cakes of just roses. THAT, I don't care for, but if it's the customer's request, then that's what I'll do for them.
As Indydebi pointed out, Thanh Thanh's sheet cakes are perfection, and I've learned through her photos that a sheet cake can be just as unique as a tiered cake.