Relay contacts tend to spark when they engage each other. You can get rid of most of that sparking by adding diodes but that would be cheating (semiconductors again), to a lesser extent using small neon bulbs across the coils (that will still take them up to 90V though).
With 'practical' I meant that you would be able to run programs to completion for for instance numerical problems.
Vacuum tube computers suffer from different problems (such as tube filaments burning out) and I really wonder whether a well designed vacuum tube computer would be more or less reliable than a well designed relay based computer.
Vacuum tubes got awfully small in their final days but the voltages involved and the heat make me believe that relays would probably be more reliable, but likely also more expensive and much slower.
