I think that randori is probably the only safe way you can practise in a unpredictable environment.
I have competition to thank for something - I am going to train in Japan in July for six months and the only reason I have the money to go is sponsorship for a competition there.
The reason I don't think sparring is worthwhile is based largely on experience. I used to do wado-ryo karate and for a period of about five years I used to go back to that dojo and practise the aikido I had learned against the karate people there. The real problem is that an experienced karate practitioner is not linear in their thinking and tenkan becomes very difficult. Also they will strike whilst backing away (a habit from competition) so irimi is very difficult. The only success I have had is only with commited attacks and in sparring thqat is not what you get.
I think that a lot of self-defense ability depends on a few simple things. I am a firm beliver in basics. I think that all styles of aikido have some form of base excercises (kihon dosa?) and it is these that help you entry speed, protect your centre line, learn weight shift etc probably the only things you can remember when in a panic!