The importance of redundancy grows exponentially with two factors:
1) The size and weight of the model (more capacity to produce harm and liability in the event of a loss of control).
1) The cost of the model (more investment to protect against crash).

Yes, you can do it for smaller and less costly models, but the price to pay is the extra weight.
As mentioned above, that is your choice, and it is not easy to find light small capacity batteries nowadays (working in parallel they can have half the capacity than one single battery).

There is an intermediate solution for small models, which is using a five cells battery and two switches.
Any radio system can work with 4.8 volts and 6.0 volts, and a battery fails one cell at a time, other than due to low charge.

Copied from

http://www.hangtimes.com/rcbattery_faq.html

"Q: Isn’t the pack the most likely thing to fail on the airplane? Why back up the switches?

A: Whups.. looks like the Wives Tale Tag Team has been busy again. The likelihood of a tested, known to be in good condition and properly charged pack suddenly suffering a cell failure in flight is unlikely. Falling short of a castatrophic weld or cell interconnect casualty the two components on the aircraft most likely to fail during flight operations are the connectors or the switch assembly; both items being very susceptible to vibration and fuel soaking damage leading to an ‘intermittent’ operating condition.. something that’s insidiously difficult to detect before it costs you a plane.

Q: What can I do to protect myself against the ‘unlikely’ event of a cell failure?

A: Cells typically fail ‘shorted’.. that means that falling short of a major mechanical failure of the battery pack cell interconnects (why you should NEVER fly a ‘soldered’ pack), electrical continuity of the system remains intact.. in other words, if a cell shorts out, the system voltage drops by the value of the failed cell. To avoid allowing the voltage drop of a failed cell to threaten the system, just use a 5-cell pack instead of a 4-cell pack. To avoid the remote possibility of a weld or battery interconnect strap failure, using parallel packs does the trick & covers all the bases."

In my experience, I have seen more crashes due to improper charging and overlooking of pre-flight check outs, than due to battery failure.
Faulty switches and loose or dirty connections (in battery, servos, receiver) have claimed another good number of victims.

Here it is, another cheap insurance against loose connectors:

http://www3.towerhobbies.com/cgi-bin...?&I=LXHVC9&P=7