With 3.10.61-rt65-1-rt-lts and my TerraTec and RME PCI/PCIe cards connected snd_seq_midi is missing, but it's not needed and all cards are shown by aconnect -i. If I connect an USB Korg nanoKONTROL and a swissonic USB MIDI IO, the USB drivers and snd_seq_midi get loaded. If I modprobe -r snd_seq_midi and disconnect and connect one USB device, snd_seq_midi gets loaded again. If I repeat it several time, then after a while snd_seq_midi doesn't get loaded anymore, but if I don't modprobe -r snd_usb_audio, then aconnect -i still lists all MIDI devices, including the USB devices, even without snd_seq_midi.