In general, no. Why would it be? It's the smart thing to do.

As long as the routes are ones where one-way fares are 1/2 the r/t fares, then who cares? If not, then you're still okay if A-B, C-D can be done as an open jaw Translation: be careful with international flights.

Price the purchase now (B-C and A-B,C-D) and see if the price is substantially more than A-B-C-D. If not, you're probably fine.