But if I'm forced to connect a "real" switch anyway I can get all the ports i need from the switch.
Want to connect three ISPs? Configure three ports on your switch on separate vlans and pass them through a "WAN" trunk to your router.
Want multiple local networks? Separate them on your switch and pass a "LAN" trunk to your router.
Need more bandwidth? Use bonding or 10 GbE, in case of RB1100AHx4 the CPU is limited to 2.5 Gbps per switch group anyway.
=> 2-3 ports in use, 10 "useless" ports remaining.
Yes, there are ways to use that many ports without switch features, but I don't think they are reasonable. I'd prefer either a router with fewer but possibly faster ports, or a CRS3xx like switch but with fast routing performance.