Here is what you need to consider when using TBS:
1. The set itself gives you ONE mundus buff. It doesn't give you two. When comparing to other options you don't get to count both.
2. The synergy between thief and shadow is why people would choose TBS for PvE dps. You are choosing wrong mundus (yes, wrong) if you use anything else for PvE dps.
3. There is an extensive discussion of TBS vs. Julianos for magicka DPS on TamrielFoundry.
4. People choose TBS for the flexibility, not raw power. For example: I can go PvE dps by choosing thief and mundus, or I can switch around for PvP. Considering the expense of v16 gear, this isn't such a bad approach anymore.