10 years would be a short sentence for one rape
It depends on the details. Under the current sentencing guidelines the sentence for a single rape can be lower than that in the least serious cases, although such cases are rare. However, if Worboys had been convicted after the current guidelines were introduced I would have expected a longer sentence than 10 years given the range of offences of which he was convicted.
PricklyBall
Whether a letter writing campaign is worthwhile depends on your aim. If you want to change the law or the sentencing guidelines feel free, but note that they have both changed since he was imprisoned. Changes are not retrospective so his sentence was not affected.
However, if you want him returned to prison it would be a waste of time. Neither the Home Secretary nor the Justice Secretary has the power to overrule the parole board and return him to prison. Politicians lost the power to prevent prisoners being released in 2002 when the Court of Appeal ruled that this was a breach of human rights.
Regarding the allegations that have surfaced since the original conviction, it is not clear why he was not prosecuted. Whether he can now be prosecuted for these offences depends on what happened. If he has not been charged with these offences and the CPS believe there is enough evidence to give a reasonable chance of successful prosecution they could proceed. However, the problem may be getting a fair trial given the publicity around this case.
I normally confine myself to giving facts on threads about criminal cases but in this case I am going to give an opinion. The original investigation was clearly botched. I am unsure whether the 8 years minimum set was low at the time but I would expect someone convicted of the same offences to get a longer sentence today. I can't comment on whether the parole board is right that he no longer poses a threat to women. But I think he should still be behind bars. I do not think that 10 years in jail is long enough for the offences of which he was convicted. If he had been convicted of all the offences he is alleged to have committed I would have hoped he would remain behind bars for life.