Yes, if you intend to use them all however you can have a second account holder to 'share' them with too. There is also a supplement for peak etc crossings but if you don't intend to go at these times then they are a good choice.
However, if you have some Tesco 'deals' vouches you can exchange these for travel vouchers at 3 x face value, which is what we do, so a return crossing usually costs us no more than £50 in vouchers, as opposed to £110 with the EFTVs.
If your going to travel five times a year works out at £549 a year , £108 per trip. I only go three times a year, twice booked with the C&CC saving 10% and once with eurotunnel directly, so this year I have paid £530 so I have paid 176 per trip over the year. So on this basis I would get two free crossings a year.