Horses for courses.
I have recently been thinking a lot about this subject. I have a twin rating and am working on my IR.
The main reason for going the IR route (already having an IMC rating) is to make touring to the near continent less weather dependable.
In that scenario only a twin will give the added ability.
Even allowing for the lack of professional retraining, higher allowances for Wx etc I think that if you fly a SE machine you will have too much lack of redundancy of systems to make IFR operations a safe option.
As mentioned here before the 2nd engine does not make an engine failure twice as likely.
There is an interesting review on fatal accidents in Senecas on the pplir website.
www.pplir.org IIRC.
To my simple mind the difference between a single and a twin is the following.
Single easier operation in critical flight phase but longer exposure to failure. Twin more difficult operation in critical flight phase but failure exposure a lot shorter.
Either scenario can be enhanced (or aggravated) by some careful pre mission planning.
I.e. Dont fly the twin near gross; make sure you are current; choose sensible Wx minimums etc.
Those who have read the AAIB report about the Titan crash up north will have had the same chilling shivers along their spine as I did when you realise that even highly experienced and trained crew can be involved in an engine out accident like that one.
MHO of course
Frank