That's not true at all. Even with an heavily overclocked modern i7 like a 4790K or 5820K the CPU will bottleneck a SLI/Crossfire-Setup in quite a few games (1.080p). In fact a lot of games will already be bottlenecked by one GPU. Some examples were CPUs are bottlenecking: Battlefield 4 MP (if your not using Mantle), Star Citizen (at about 50 Fps once other ships are around, down to 35 in New Horizon Racing), Star Craft 2 (even in 4K), most MMORPGs, most open world games like Watch Dogs (50-70 Fps), Grid Autosport (about 80-100 Fps) and a lot more.
SLI/Crossfire will only be an advantage if you up the resolution or when DX 12 is finally around, hopefully removing CPU-overhead.
Regards,
blaidd