It is wise to take out legal cover with your policy, as the additional cost is negligible. As far as home assistance cover is concerened, it all depends on what the extra premium is. Companies such as Npower are constantly trying to sell policies covering some of the same risks, but not a whole package. If your house is getting on in years it may be worth taking out the extra cover.
Home Emergency is a pretty good addittion (depending on how much extra --- the company I work for has a premium of just under £3 a month) this covers you for replacement keys, locksmiths etc
Legal is also a good addition (extra premium just under £2) and covers all sort of disputes, bodily injury and so on
Guess it all comes down to peace of mind....you hope you never have to use them, but if you do, the help and advice is there